Try politeness first: ask the unwanted person to leave, explaining that he is intruding on a private party. If your intruder refuses, tell him to leave. If he still refuses, phone the police. When the police arrive, repeat your request in their presence. Only then can they act to remove your intruder.
Avoid physical confrontation. Enlist the help of a guest in keeping the gate-crasher quiet and away from the rest of the party. If he is drunk, phone for a cab. If he insists on driving, immediately contact the police with the registration number, make of car and location.
