I don't know about you guys but I'd be pissed if someone boxed my car in like that. My first reaction would be to tell him to get out from behind my car. If he refused, I'd have to assume he had some kind of ill intention..
I've found that with most pan handlers, a simple threat that you're calling the cops will usually cause them to haul ass from the area. That would be my next step if he refused to move his car.
Of course, if he refused to move and then approached me after I asked him to leave me alone, then I'm going to have to assume he's a threat to me and he's about to attack.