Now you are getting all defensive.
If you aren't interested in constructive criticism why did you post your experience?
Your option is to say no thank you and go on about your business. Your "rights' do not trump anyone else's right. To paraphrase you, I think part of the problem is we tend to deny other people's rights and claim it as ours and only ours.
Can't be disrupting other people's ability to be anywhere they have every right to be. Which can be the strongest point of leverage a potential assailant has for getting inside our defenses.
All we can do is to remain aware and prepared to the degree possible, being capable of exploding as violently and ruthlessly as needed in order to redirect and thwart an attack if it occurs. Anything more puts us into the hot seat, legally speaking, if we're not careful.
He might have a right to stand in a spot, but there's no requirement I stop and talk, and thus let down my guard. Experienced panhandlers learn to prey on your socialized response. Learn to break that pattern if necessary.
It is a common technique called stacking, where when fighting multiple opponents you maneuver and line them up IE; stack them to give yourself the tactical advantage. He had you and dad stacked in the cab by your own actions. The suggestion to exit the cab was valid, allowing you the freedom of movement to deal with the situation.
"Sorry, can't help you today," has always produced an amicable repy and a walk-away.
Good thing you and your dad were alert. It's a shame that you can't trust anyone nowadays.
A very firm, "I can't help you" does not have to be rude but can change the dynamics of the situation instantly. If they continue you can tell them to "stay back" and you can still move to put something between you and the person or put yourself in a more advantagous position. You don't have to curse at someone to let them know not to mess with you. Don't look to Hollywood for any tactical answers.
Glad everything turned out ok. Hindsight being 20/20 and all one thing I think you could have done to help avoid the problem in the first place, would have been to not avoid the guy. By getting in the truck first you avoided the guy but left your father as being potentially more vulnerable. Wether he wanted to cause a problem or just panhandle most people would be less likely to get aggressive if the odds aren't in they're favor. Even though you were still there for your father and were a threat if this guy tried to do something you probably didn't appear like a threat any longer to him. And thats possibly why he went to your dad. Not explaining well but hope that makes sense.