I'm afraid that as long as your Plan's prolific letter writer has the time to persist, there's not much that your E/C and Strata Manager (SM) can do but respond; that's the proper thing to do, but the nature of those responses is very much within E/C's control.
Upon receipt of his next letter have your SM respond, and include a para. advising that all future contact with your Plan's Insurer and Contractors must be via the SM or E/C Secretary (depending upon the delegations under your Agreement), and that any and all matters to do with the management of your Plan are best raised personally at E/C or General Meetings.
After that, if the correspondence continues (and it may well), I'd suggest a very brief shell-letter type response, that simply acknowledges his. Once your serial complainer realises that that's all he's going to get (e.g. no apparent reaction), but that the door's not shut (because it can't be), he may attend personally at Meetings where you can better control what he says and when.
Defamation is largely in the eyes of the reader, so just ignore it.