Wedding guests inviting additional guests?

Dear Miss Smarty Pants:
The response cards in my wedding invitations had a space for "We have reserved ___ of seats in your honor." In the case of one family, I put the number 5. I even wrote in all 5 individual names to avoid confusion and make it clear that guests were not exchangable if one we intended to invite could not make it. When they mailed in the RSVP card, they crossed out the number 5 and replaced it with a 7, writing in the names of the two extra guests that were not invited. What do I do?

You write them a note that reads as follows:

I'm terribly sorry but due to budget constraints, we are not able to allow extra guests. I must request that only the the 5 original invited guests attend. I'm sure you will understand that in this economy, I've had to make very hard decisions about whom I am able to invite and I cannot allow any of my guests to add people to the guest list as I am limited to the number of people that I have contracted for with the caterer and the venue. I look forward to seeing the 5 members of your family that received the original invitation and no more can be added. Thank you for understanding.

If they are upset and decline the invitation, it frees up 5 spots on the guest list. It is the height of rudeness for them to have done this and do not tolerate it or let them get away with it. This is YOUR wedding and YOU decide whom you wish to have present!