I'd urge you to reconsider.

I can understand why you'd do that, but honestly I don't think it'll help much. Anyone who wants to "game the system" will probably just use a couple throw-away addresses from a disposable address service or something similar.

Further, blocking the use of that character is an inconvenience to people like myself who use it to track where their mail comes from (i.e. if I get spam to 'mybox+launchrock@example.com', I know exactly who leaked/sold my address.)