I do bits and pieces of what has already been proposed.

In my opinion there are two kinds of whataboutism: valid and invalid. Valid requires some argument to rebut, but fielding invalid whataboutisms are easy.

Whataboutisms are used to deflect criticizing something that someone holds dear to themselves. So there is an inherent sense of emotion going on that becomes harder to deal with rationally.

In the case of a valid one, someone else gave a great example about bees and pollination - I agree that this is a valid use of whataboutism. In the realm of software, you can say "well, what about this?" and it's related, a related system, a potential flaw in the code. It is being applied correctly and requires a rational diligent rebuttal.

In the case of an invalid one, like primarily right wing people burning Nike shoes deflecting with the left supporting sweat shops. This one has several possible rebuttals, but the patterns are always the same, in my opinion.

"Why are you defending the crappy thing you did by talking about a crappy thing possibly under my umbrella?"

e.g. You're burning shoes and your defense of that is that I approve of sweat shops? I do not approve of sweat shops, but you approve of burning shoes. Cool deflection, bro. Also, you owned the shoes, so you supported sweatshops, too. We both had similar stances on that as Nike owners, prior to this debacle. Sadly, a lot of people buy Nikes (regardless of right/left) that have heard of their sweatshop problems. That's like saying "I burn shoes, but you're human being." We're both human beings. We're were both human beings before you started burning your shoes, so this is a confusing stone to throw because it also hits your own glass house. "I'm sorry officer, my minor speeding isn't an issue because of all those unarmed black people you guys shoot." That's not how it works. I don't get out of my speeding ticket because some police do bad things. Also an easy path here is that you could say that The Left (or whatever) is not a Hive Mind and people are capable of independent thought apart from the group.

"I agree with you, sweatshops are bad, but burning shoes you already paid for is still moronic."

e.g. Agree with the 'moral' theme of their unrelated counter-point (this counter point usually comes from emotions). Don't agree with the specific point. In this case, "I agree that people are being crazy about Nike right now. I wish we could all look at this problem more rationally."

"I don't do/support that."

e.g. Immediately falsify their counter-point (not always possible). If the police pull you over for speeding and you "whatabout" unarmed black men being shot, hopefully the police will quickly say, "I don't do that, nice try. Here's your ticket." Obviously in this case, if you are a cop that has shot an unarmed black men, you can't honestly argue this point, so don't.

Counter-troll them.

e.g. Whataboutism applied invalidly is almost always used to troll someone. Historically speaking, that's what it has been used for. Troll them back. They may quit it. You will have mixed results here. Remember, it's about morality. Burning shoes? What about those sweatshops, liberal? Oh, you're into child human rights now, what about helping out with those disgusting and immoral Baby Jails? This is a messy path to go down, but if they start getting flustered you might point out that it's best to defend your points rather than say your crappy thing is ok because something else crappier is going on. Make them defend shoe burning without pointing the finger. If I made a really good cookie, I should defend it on other merits than all the other cookie manufacturers suck.

Conclusion:

It has become easier for me to fight whataboutisms using one or more of these in a given conversation. Some of these could escalate emotions during an already emotional irrational conversation. That's always a risk when you're in the realm of unethical and irrational arguments, which it almost certainly becomes, once someone busts out an invalid whataboutism.