We can't stop them from forking it, sure. But just because the license allows it doesn't mean we have to enable the facilities to do it on the page.
We don't delete projects,we just ship them off to CurseForge land if we don't want them on WoWAce.
In any case, no fork has any chance of succeeding, imho. I will definitely not allow it on WoWAce (we do have a project acceptance policy in place here), and some Ace2 fork on CF does not stand much chance of being popular. Considering all projects still using Ace2 are pretty much abandoned, there isn't much chance of those projects actually using that fork, anyway.