I think you are on the right track, Beansxz, when you say that people are against anything new or different. People do
fear and dislike what they don't know about/understand. Unfortunately, sometimes this misunderstaning displays itself in vulgar behavior, such as giving the finger, etc.
As a responsible traceur, it is YOUR job to be sure that the public knows otherwise. The worst
thing you could do is to respond to someone's inappropriate behavior with more inappropriate behavior. The best thing to do is just ignore it
Instead, find other ways to improve (or even just introduce) parkour's image in the public eye. For example, you could form a parkour group and have the local news do a story on it. (*Note: Be VERY careful about this and consult someone in the parkour community that has experience dealing with the media first. A bad report on parkour could do more damage than help.) Or you could have monthly litter-pick-up days or some other community service type activity to show the public that your parkour group is serious about being respectful to the community and environment.
Lastly, I don't know how long you have been doing parkour, but if you are at a level where you are qualified to instruct others, you could start a community parkour program, to teach others the basics in a safe, fun environment.
The general rule of thumb is basically to NEVER act "in the moment" on your emotions/reactions. While you are practicing parkour, whether you realize it or not, you are representing the ENTIRE parkour community to those around you. Represent it well