I've been annoyed by a few members on COMC who keep sending me the same lowball offers, but I've dealt with the same thing on eBay for a long time since I use the BIN/Best Offer system over there. I'll counter the first lowball, then reject the second.
I think a good solution for COMC would be to implement the same offer limit that eBay has. A buyer has a limit of 3 offers on an item. After they hit that limit, they can't annoy you anymore.