Tell him to stop nicely once. If he doesn't stop get up and move away from him. If he tries it again, don't be so nice - say stop louder- so maybe someone else will hear. Then get up and leave. A guy who doesn't listen when you say no ( or a girl who doesn't listen) is not a person you want to be with.
If this person is not your boyfriend and the person trying it is somebody you do not like OR it is a person you do not like sexually - they yell stop touching me so all the world can hear. Then go tell somebody who can help you. If the first person you tell doesn't listen - go tell someone else. Keep telling until somebody listens.
Touching you when you do not want to be touched is sexual abuse.
If you need more help with this - send me an email.