Thanx for the advice. I will tell somebody and I will stay away from this guy. I'll keep you posted.
Good luck and I'll second the advice given by nearly everyone. As Marguerite Nightingale noted, please take a minute to write down a list of each time this person bothered you (date, time, place, and a description of what was done/said) plus what you have done about it (what you told others or the person) and their reaction. This will help you explain what's happening as you talk to people and will also provide a good evidence trail should that be needed.
Most likely the people you talk to (your parents, the instructor at the dojo, etc.) will be very supportive of you. If they blow you off then please talk to a teacher or administrator at school, as they will know how to get the wheels in motion to get this stopped.
It's definitely not ok that someone is bothering you.