As she stood at the touching ceremony to rename a Brussels square Place Jo Cox, the murdered MP’s sister, Kim Leadbeater, remembered her hope that the assassination of a principled politician would warn the British of the dangers of violent extremism. Fat chance. In a voice that spoke to her and her country’s unhappiness, she said: “Despite many people working extremely hard to show that Jo’s murder was not in vain, I’m sadly not at all sure that this was the case.”

The fight against lethal bigotry is as feeble as the fight against money laundering. Whether they are banks handling looted resources or Silicon Valley companies resisting demands to spend a portion of their profits on hiring editors to moderate content, the guilty parties can rely on governments to regulate them with the lightest of touches.

At the level of traditional policing, few can fault the state’s vigilance. The British neo-Nazi outfit National Action celebrated Jo Cox’s murder – “only 640 MPs to go”, it declared. The government banned it and in July a judge jailed two of its members for plotting the murder of another Labour woman, Rosie Cooper MP. In a pattern familiar to watchers of Islamist groups, National Action rebranded itself as NS131 (National Socialist Anti-Capitalist Action) and the government had to ban the reincarnation too.

Yet as far as the online world is concerned, NS131 remains an active propagandist. Google it and you reach its site as you can reach the sites of Bomb Islam, God Hates Fags, the American Nazi party and many another incendiary organisation.

National Action is guarded by the San Francisco company Cloudflare, which serves about 100,000 of the million most-visited sites. The first amendment to the US constitution gives vast protections to freedom of publication, which are extended to the extremists whose content Cloudflare holds. So vigorously does it seek their business that Wired magazine described Cloudflare as “acting like the muscle guarding the podium at a Nazi rally”. No global tech conglomerate can operate with a base in just one country, however.

To speed up downloads, Cloudflare has built “edge servers” – data centres that store content locally. There are 30 in Europe, including one in London and one in Manchester. The British government cannot regulate the worldwide web, but it could enforce the law in Britain. The anti-fascists at Hope not Hate begged ministers to make Cloudflare’s British operations comply with anti-Nazi legislation. The Home Office did not even have the courtesy to reply.

To pursue the financial comparison, the state will arrest small-time crooks but won’t confiscate the assets of Russian oligarchs and the the the lawyers and accountants who serve them. Equally, it will arrest local fascists but it won’t go after the tech giants that enable them. Determined action would not contravene basic principles. The liberal conception of freedom of speech holds that it should be defended until it incites violence. So many of today’s no-platform bans are objectionable because “Terfs” and other targets of woke anger are not inciting violence but having an argument. Cloudflare, by contrast, is enabling men who want to kill, not argue.

Tech pushes them along with millions of almost imperceptible nudges. A shocking sight at last week’s Labour conference was Luciana Berger walking with a police escort. As a centre-left female MP, she is a target of the far right. As a Jew, she is a target of the far right and far left. More shocking to my mind were the assertions of Corbyn’s Twitter fans that the police presence was just a “stunt” for “the cameras”.

Corbyn did nothing to restrain them. After being shown pictures of her bodyguard, he blandly intoned: “There is no threat being made in this conference”, despite the fact that men have been jailed for threatening Berger and despite the appearance of a picture surfacing of Corbyn grinning alongside a Labour supporter whose definition of free debate was “the Holocaust – yes or no”. Before Corbyn’s claque starts squawking, I should say that, if I am to accept your assertion that “Jeremy has fought racism all his life”, you must explain how he became the world’s unluckiest anti-fascist: a a 21st century Forrest Gump, who keeps finding himself alongside men who would drive their populations to starvation, oppress women, kill Jews and gays and torture prisoners of conscience. Through no fault of his own?

The conspiracy theories dismissing clear threats to leftwing women are spread by Twitter; the narratives of radical Islam, Holocaust denial and anti-Muslim hatred are spread by Google, Facebook and YouTube. So essential have they become that the mass murder of Myanmar’s Rohingya Muslims earned the bleak distinction of being the first genocide whose executioners incited each other on Facebook .

You might say I make too much of technology’s power. The genocides of the late 20th century were committed in Saddam’s Iraq, Milosevic’s Yugoslavia, Rwanda and Darfur without the aid of wifi. The communists and fascists of the 1930s needed no microchips to organise their camps.

A comparison with pornography makes the case against complacency. When the web began to spread porn round the world in the 1990s, self-aware men noticed a strange phenomenon. Users who flicked through the expanding libraries of sexual titillation found themselves aroused by fetishes they never knew they had. Maybe realisation would have dawned in any event: kinky sex did not begin in 1993, after all. Yet Nineties men could ask whether, if the internet had never existed, they might have lived their lives without the smallest idea that they had a predilection for dominatrixes or golden showers.

As it was, technology gave them pornography that 10 years earlier could only have been obtained at enormous expense from Soho basements. What applies to sex applies to violence. Once, the propaganda for radical Islam and the British far left and right was also confined to dingy bookshops. Now it is available everywhere, we have a duty to combat it everywhere.

• Nick Cohen is an Observer columnist