I am a PhD student and hate it here. How can I warn prospective students during admit weekend without ruining my reputation?
Focus on facts, rather than your emotional impressions.
Don't try to overgeneralize. Say what didn't work for you, rather than imply that it does not work for everyone (unless everyone agrees) or it won't work for the prospective student (who knows...).
You could say "I would have chosen differently" (ideally, adding the other possible options) or "overall, I dislike this program" rather than "this program sucks" or going into long rants.