It may well not be trolls. It may well be someone who’s fed up of seeing language partner requests on the discussion part of the forum. There could also be other reasons.
In regards to knowing who has liked or disliked a discussion, I can see obvious problems for those users who are not trolls, eg harassment.
A troll really doesn’t care. Their identity is normally never real and their sole function is to cause hurt and anarchy. So revealing them will make no difference, because we don’t know who they really are, plus they’ll probably setup another account.
Italki has to create a separate discussion forum for people looking for a language exchange partner. That may reduce a user downvoting and reveal the troll, because I’m assuming Italki knows which account downvoted a discussion.