I've handled it just as JLyon suggests. Retired at 5-1. No sense getting your own rating unfairly boosted. It's not like it's going to be an enjoyable match to continue. Very effective.
I've also seen captains (in a non-Sectionals-track league) band together and decline to play teams where this was their modus operandi. Very satisfying to see. Much more effective than filing grievances, though that's still an OK (though probably futile) idea after the fact so as to build up an official track record on the guy.