This is an online forum that practices gardening. There are lots of other online forums where you can speak freely. Oh, but you'd rather speak here, to the people who gather here to listen? Well - maybe they're gathering here in this garden because they don't want to be in those other ungardened forums, and so no, you can't speak freely.

My personal guess would be that they are gathering here because many seeds are blown here (from HPMoR, for the usefulness of the sequences and other materials), and the gardening does not have a significant net positive impact. This is mostly based on the fact that I have spent time on ungardened forums and I have never felt that the lack of gardening was a real problem. Where they declined it was because the inflow of members died down, not because members were turned off by trolls.

I'm not sure there's any evidence out there to settle this one way or the other, and certainly in the end you can do whatever you want, but I think you should at least take note that what seems to be a significant part of the userbase thinks strict moderation does more harm than good.