What the previous posters have said is correct. Pulling a fake gun on someone is just as bad as pulling a real one and not actually shooting in a lot of places...it's called "brandishing" and in some places is a serious crime.
you have four real options if this really is a serious problem...in increasing order of risk
first, and most desirable, avoid the areas you are getting harassed. If its an area where there are undesirable types hanging around, it's understandable that they are harassing you. I live real close to the bad parts of Oakland CA and Richmond CA (two of the most crime ridden areas of the US) and no way are you catching me on a bike there.
second, carry pepper spray or similar type thing.
third, learn to fight.
fourth, carry a real gun. I find this option silly for cycling, but I commonly carry a large handgun when I go hiking in pot farm or bear country so I can't totally dismiss it or it would be the pot calling the kettle black.