Sounds to me like you will have to start hiring a bouncer! LOL!
I would make it clear that people need to bring the invitation with them....and have a list at the door....and when a rude person calls tell them..."Ooooh! So sorry! We are at our limit for guests."

LOL. Tell all the uninvited that it's a 'private' party !!

I think if it is a formerly loyal customer whom might send you some business again, you may need to suck it up and let them come. But when they call, I'd certainly make the point that they may have been automatically removed from your database since you can only invite current customers, oh and by the way do you guys have any events coming up sort of thing.

I'd definitely spell out on the invite that this event is a thank you to our cutomers who have made the past year a success/invitation required for entry.

The nature of your business probably means you can't alienate people too much, but if they cross that line to just being a mooch, I would absolutely use the "limited parking" type lines suggested by the others.

Good luck!

I wouldn't use the limited parking excuse, they just might get a ride to the event and be dropped off.

Tell them that there is a certain amount of customers that you and your husband can host which you have already met.