I work in real estate and although I haven't done a lot with HOA's a know a little about them. You did the right thing by starting with the property managment company. You should pull out a copy of your HOA bylaws and see what they say. Usually such notices would come from the managment company, this is a good example of why the HOA hired them, to prevent this kind of problem. Write the other members of the board (sent to their personal addresses) and politely ask them to ask this person to recuse himself in future disputes as the two of you have a personal problem that has nothing to do with the HOA.
If he keeps it up, PM me and we can talk about what's next.