I work in DC, just outside of Gtown, but also i used to work in Gtown as well. and yes, parking is a mutha down there. Entitled people thinking they can do what they please.
for the hitch, simple, i just left on a spare trailer hitch arm, no ball. Because even if they dont bump you, they have no problem leaving you ZERO space to back up and pull out.
for the front, thats tricky...who wants to mess up their front bumper. I'd go with finding another garage if you can, looking for an outdoor parking lot of some kind that might take a monthly rate, or one of those devices like the thicker license plate holder that protects your bumper some what.
you could also always go with a steel bumper...someone backs into you with that, they will only be looking to pay for their own repairs.
Some people...what can you do...