What to do?

I have a problem which I have decided to solve by doing nothing. I don't think it will work.

We live in a duplex -- my roommates and I share the 3-bedroom upper unit and our neighbors share a 2-bedroom lower unit. There are 2 parking spaces in the building's carport and and a space in front that is small, but can hold a parked Honda Civic, like mine.

The downstairs neighbors moved in because my old roommate knew them and told them about the place, so we know them a little, although I know them the least of everyone. Since they moved in in January, we shared the 2 carport spaces -- one each -- and our unit dominated the front space saying we had always had it. Occassionally though, the short fat one parks his truck -- which only fits by blocking in his roommate -- in "my" space.

The facts: No lease, neither ours nor theirs, discusses the parking assignments. We have a 3-bedroom vs. their 2-bedroom. We pay more rent because of this. 98% of the time, the space is mine. He only parks their when the parking on the street is bad.

I'm afraid, though, that this trend is becoming more common. What should I do?

Should I say something to him? Should I demand he stop parking in my space? Should I create a schedule which we use to share the space? Should I slash his tires and his on his upholstery?

Help!