@branadian13: In which case, the broken record method works best.
“I am sorry but that won’t be possible”. Don’t offer them an explanation, they aren’t owed one, and will try to work around whatever reason you provide.
Them: So Jim Bob can’t make it to your wedding, so I thought I would bring Jinger instead
You: Oh I’m sorry Michelle, there has been a misunderstanding. The invitation was only for you and Jim Bob. It won’t be possible to accomodate Jinger at the wedding. Can’t wait to see you though.
Them: But Jinger loves weddings, and since Jim Bob can’t come, I don’t see what the big deal is.
You: I’m sorry, like I said that won’t be possible.
Them: But….whatever other stupid reason.
You: I’m sorry that won’t be possible.
At this point you disengage from the conversation.
You: Sorry Michelle, I’ve got to run, the cat’s on fire. See you soon. Bye.
As for guests who don’t RSVP, a polite host waters down the soup, and quickly sets and extra place if an extra guest comes. When your RSVP date passes, call up the non-responders and ask if they are coming. Don’t email them, don’t facebook them. Call them on the phone. Leave a message if you can’t reach them.
I operate on a 3 strikes policy. I will call you and leave 3 messages. In the last message I will leave them a deadline to respond, with a “if I don’t hear from you by Monday, I will assume that you cannot make it., You will be missed.”
This should get you most of your guests.