What should I do if a paper makes false claims about my work? I recently found a paper published in the proceedings of a very important AI conference that makes a false claim about my work, i.e., claim that is false from the technical point of view.

I already contacted all the authors when I found the paper on arXiv, kindly explaining that they may have misunderstood my work, and asking to adjust that claim. They never replied and now the paper is published in a quite good venue.

Should I try to contact them again, or what?
That is what the editor or technical chair of the venue is for: to act as an independent moderator. Contact them, clearly and briefly explain the situation and indicate that you previously contacted the authors before publication but received no reply. 

With overzealous spam filters, there is always a chance a reply came but was blocked. Start a constructive dialog so that the correct facts can be brought out in the correct publication (maybe yours will be revised to specify clear limits of applicability). It's a slow process to resolve such things, but it is certainly doable.