Tricky one, Often when the whole thread can be in effect deleted by the original poster, what tends to happen is everyone jumps the gun and assumes mods deleted it. You then get new posts along the lines of "where did topic x go? why was it deleted?", followed by people complaining.

Maybe a bit of both is the solution? Delete the original post content, keep the subject line and all the replies up but close it off for further comment?

I'd go for deleting the lot, simpler that way.