set %variablename $true
But you will have to decide when to unset it; otherwise, the next time you WANT it to send the message again, it won't send it.
For instance, you might want to unset the variable when they part the channel or quit IRC. If you have variables named %greet.Hammer.#mIRC you might unset them like this:
on *:PART:#: .unset $+(%,greet.,$nick,.,$chan)
on !*:QUIT: .unset $+(%,greet.,$nick,.*)
