How many times does he have to do it? He's already done so a number of times. It seems like the answer is he will never be able to satisfy people.
I've not been following all this on Twitter well enough to know all the details, but I do think there are specific things he could do, like make it clear to people on his official campaign staff that some things aren't acceptable and will lead to dismissal, and follow through on that.
Similarly, he could disavow that Chapo podcast's vile attack on Elizabeth Warren after she pulled out, and make it clear he doesn't that sort of support.
However, my point -- I wouldn't presume to speak for Dakota -- is more to do with how his activist supporters behave online and in meatspace.
That is, assuming they are actually out there campaigning and canvassing for him, I'd hope that they'll be out there canvassing and campaigning for him come November, too, and I assume they'll expect Democratic activists who've supported other candidates -- Warren, Biden and so on -- to do the same.
My point is that they're not actually making that any easier for people -- we've talking about volunteers here, after all, and people aren't going to find it so easy to work alongside people who've been abusing their preferred candidate and the candidates supporters (so, by extension, them) so roundly only a few months before.
That's why I hope people on both sides will try to remember that they're going to need each other come November, no matter what their disputes now, so they should try not to make the situation any more difficult than it need be.