Oooooh I hate this too! Especially with something like a wedding that you are paying for per head.
A couple of DH's family WOULDN'T RSVP to our wedding as they didn't know what the weather was going to be!
I made it clear that with DS' upcoming birthday party that if they didn't RSVP, I will be assuming they're not coming. Guess who still hasn't RSVPed?