I hate such behaviour and have been railing against it in particular in live games where its even more common. It's poor sportsmanship, poor behaviour, and all around not fun nor legitimate.
There are three (very soft) methods around this I've seen:
-Password protected games with people who seem or known to be trustworthy. Hard and tedious to set up, and near impossible for live games.
-Allow public messaging to communicate draws/cancels. However, this is not true gunboat and there's nothing to stop it from becoming an ACTUAL public press game, which isn't the worst thing I suppose.
-Play games with bets of 101

or more to set a high bar of entry, the idea is two fold: 1) only players who are relatively experienced and consistent should have enough to join 2) with such a large pot the temptation to NMR willingly is drastically lowered. Of course, not all of us have that many points (yet) to throw around...
None of these are particularly good solutions.
I came here to play Diplomacy, but the amount of times the game I end up playing is NOT Diplomacy, but a bastardization, is unfortunate.