I like this, but it would involve me quitting every single time I see it coming. My online record (it isn't impressive by any means I admit) would get even worse
Nah, if you hit quit instead of surrender, it doesn't count as a loss. The only stat that really counts is number of games played.
and I further suspect word would start getting around about me that I'll quit after 30 minutes.
One thing you can do is have 5 different accounts to play on, and alternate between them. If "word starts getting around" you could just delete one of the accounts and make a new one.
But the main thing is, who cares if word gets around? Word would only be getting around from seeker/discple spammers to other seeker/discple spammers. Do you care what they think? What would they say about you, anyway? That they don't have fun playing with you because you refuse to let them pummel you with an OP strat? That you're no fun because you're not a masochist and don't enjoy pain?
The title of your thread is that you are quitting Sins online anyway. So if you stay and try my counter, the absolute worst that happens is you get no games, that word gets out, blah blah, which makes you absolutely no worse off than if you quit Sins like you plan to do. Of course, it's your game, I really don't care what you do. But this is my official counter from now on.
Another thing I've thought of doing is hosting games that say "no advent" or something like that. Of course, as you point out, you are eliminating 90% of potential players, but again I say, do you really want to play with advent disciple/seeker spammers?
Anyway, good luck with whatever you figure out.