I take it these are players you want to keep around but just not as vampires? Otherwise, kill them off.
To kill them off you have to keep them talking in the sun,or invite them to a party and lock them in so they can't leave and just keep talking to them a lot. Eventually they run out of plasma fruit and die.
You can also, using testingcheats enabled, kill them off by putting them in your home lot swimming pool, remove all ladders, and seal it off with Gothic fence. It takes awhile but eventually they do die.
You can also go into edit town, find the house they are living in if they aren't some of the homeless townies, remove them from the house and your clipboard. The game will generate more vampires so it's not the best method.
Can you tell I really hate vampires?