Usually when I don't want to preform sexual favors on my boyfriend, I think of things I could say to him to kill that boner, or make it rest for a little while. But I never do. Some of the things I think about saying are:

"Hey baby, I feel a queef coming on."
"Oh God, think about it, you came out of YOUR MOTHER'S sick, hairy, vagina."
"Jesus Christ, Holy Shit, your balls feel like an elephant! That's groooooooss" (I have said this before)

I can't remember the other ones right now.