Unless a significant amount of time has passed, and the content is no longer relatively as good as it was earlier, allowing takebacks on commendations would make for choppy moments (a person is happy and then unhappy at reversal of fortunes).
That said, I know the feeling of sending out a wrong email all too well, so how about a compromise? Commendations can be cancelled within 3 days of them being issued, but not after. Gives enough of a window in case something damning about a storygame comes to light, without making a very mercurial system either.
However, under such a system, it may be wise to not notify a commendation has been awarded till the 3 day period is up. This is based on the Boomerang plugin for Gmail, which allows an email to be unsent within 10 seconds of clicking the send button (by keeping it in the outbox but not releasing it till then). I'll admit, it's saved my hide more than once.