You could remove all the installers, replace them with a README.txt, and update the website and project description.

That said, looking at the top story, I guess they consciously chose not to do that and then things went sour.

Doesn't matter. Someone elsewhere in the discussion found out what they're doing - Sourceforge now has a policy of taking over the project pages of projects that've moved off Sourceforge and running the pages themselves as mirrors (apparently with added extras in the installers): http://sourceforge.net/mirror/ If you remove all the installers, they'll just get them off your website instead. If anything, removing everything from the project page would just give them even more of an excuse to take it over.

I wonder if there are any OSS licenses with a legally valid "except for Sourceforge" addendum available.

OSS can just use trademarks to stop SF from redistributing the binaries under that specific name (thus will kill their search rank).

Good point.