Need help from the Emily Post types...!

This is a business event being hosted by a nonprofit. We invited some VIP's as our guests and clearly made the invitations individual to them only, addressed to their offices not their homes. But a couple responded back that they will attend with their wives! I think it was really rude of them to try to inflict uninvited guests on a small nonprofit with a tight budget, but how do we politely set this straight without offending important leaders in our profession?

The event should have been strongly advertised as "EMPLOYEES ONLY-NO ADDITIONAL GUESTS, PLEASE", that might have made it clearer. Another way to eliminate additional guests would have been to issue only one ticket out to the employee whose name was on the list along with a letter stating that "THIS IS NOT A FAMILY AFFAIR. NO ONE WITHOUT A TICKET WILL BE ADMITTED: EMPLOYEES ONLY, NO EXCEPTIONS". Place cards could also have been used for meal service. Unfortunately, some people do not understand some business protocols. Therefore, they must be left with no alternatives.

Unfortunately there is no polite way to tell them their spouse was not invited.. Those who are bringing the wife/husband will find out on evening of the event when none of the other guests brought a spouse that they have made a regrettable mistake and (hopefully) be more careful in the future.