I find physical intimidation is the most effective.
I get followed by skaters and such a lot.
I get a lot of little jeering comments about being a wanna-be skater without a board.
All you have to do is casually mention that you can lift more than they weigh and throw it.
Demonstrate it if need be, just do a fireman (wrestling move my friend taught me, just grab them under one leg and lift them on your shoulder (you only need one arm because your torso does the lifting not your biceps) looks really impressive considering it appears that you just lifted them with one arm.
Ive never had to fight someone over parkour, people usually just brush you off as a "dare-devil" or just some stupid kid out for kicks.