Simple solution: Drive them to the party, then leave. Turn off your phone and screw them over. They can call a cab to get home.

Call the cops on the party

So say no... you're not obligated to drive them anywhere.

How did you get yourself in that situation? Damn, that's not ok.

Then undesignate yourself. Or are you a complete doormat?

Why are you even putting up with that? Just say no; you don't need to take that.

Stop being a mug.

How are you designated driver at a party you are not attending? One of the people attending would have to be the designated driver.

"So, I'm not even allowed to attend and so much as socialize with you, but I have to drive you fuckers home? No."

This is when you say: No!