read it on bee-l a while ago and it seems a weak solution of ordinary bleach ( Sodium hydroxide + Sodium hyperchlorite, with no perfume or surfactants ) kills almost all nosema spores. If I remember rightly there was some question about "encapsulated" spores being protected from the bleach but there's certainly no residue issues with normal bleach. I couldnt find any UK info on this, so "user beware !", it might only work on the other side of the Atlantic.