"Perhaps the time has come to restrict the site to "music only" but I hope that other solutions will be found"

Not a bad idea I'd have thought. Or let BS be members-only and separate it somehow from the mother ship. While some members no doubt value the BS drop-in centre highly, the musical resources are supposedly what Mudcat's remit is about (in fact isn't Mudcat a folk-music charity of some kind?)
It would certainly tidy up the issues around 'freedom of speech' without either censoring specific non-musical topics, or banning specific individuals from participating in music discussions.

Whatever happens, I do hope the musical resource remains. If that means tightening aspects of moderation, so be it.