Nigel Farage is cracking down on Ukip supporters’ social media activity after a series of scandals over racist comments. The party has changed its constitution to prevent unauthorised use of the Ukip logo by supporters, members and officials, while Ukip’s chairman has warned those tempted to join Twitter: “My advice: just don’t.”

The move follows hugely embarrassing revelations about the publicly stated views of a host of Ukip European and local election candidates.

In one of the more notorious episodes, William Henwood, a Ukip candidate in Enfield, was exposed by this newspaper for tweeting the suggestion that Lenny Henry, who had recently lamented the lack of ethnic-minority faces on television, should emigrate to a “black country” and does “not have to live with whites”.

Andre Lampitt, the star of Ukip’s 2014 party election broadcast, was suspended for calling Ed Miliband “not British”, attacking “evil” Islam and saying Africans should “kill themselves”.

And just last week Ukip’s Kerry Smith, a prospective parliamentary candidate, resigned after recordings of him making homophobic and racist remarks were leaked. Smith was recorded describing gay people as “fucking disgusting old poofters” and a woman with a Chinese name as “chinky”.

On Friday, Farage threatened to escalate that row after describing the Essex councillor as a “rough diamond” and criticising what he called metropolitan snobbery against people from outside the capital using “colloquial” language. Challenged over Smith’s use of the word “chinky” to describe a Chinese person, Farage asked: “If you and your mates are going out for a Chinese, what would you say you were going for?”

However, Ukip’s national executive council has now passed strict new rules policing members’ social media activity and threatening immediate suspension for those who “embarrass” the party.

A copy of the new constitution, seen by the Observer, lays out “rules for online communication”, which include the diktat that “party members shall refrain from using the Ukip logo in terms of their online postings, including avatars, unless they have express written consent to do so from the party leader, the party chairman, the party secretary, the general secretary, the party director, the regional chairman or regional organiser for their region.”

In a recent edition of a Ukip members’ magazine, party chairman Steve Crowther wrote of social media: “The NEC has adopted a new set of rules for online communication to fill a notable hole in our code of discipline … My advice: just don’t. Remember life before you could delight the whole world with your every passing thought? It wasn’t so bad, was it? I have no Facebook page, Twitter account or Instagram thingy. It’s lovely.”

Duncan Cahill from the Hope not Hate campaign group said Farage’s problem was not that his candidates had previously been exposed for holding racist views but that the party continued to recruit “bigoted” people. He said: “Farage covering up the views of his candidates is not going to work, because day by day people are waking up to the sort of party that he runs. They are seeing through him.”

A spokesman for Ukip said the move had been made, in part, because imposters were using the Ukip logo on racist social media accounts in order to embarrass the party.

He said: “We have always been very relaxed in our approach to our brand. But as we grow we find that we have to be much more brand-conscious.

“It is well known that some agents provocateurs set up fake accounts and go to the press to tell them how terrible Ukip is. We need to enforce our right as the copyright holder of our own brand. The idea that this is somehow not entirely normal practice is absurd.”