So talk to them, face-to-face. Tell them that you need to be able to get your car out when you need it, so you're not going to be able to let them park behind you. While talking to them, ask them about the beer; tell them that you'd be willing to share, but not if they and/or their houseguests are going to take without asking.
I mean, I *hate* confrontations (ask my wife), and this strikes me as a situation when you need to buck up and talk to them.
I mean, I *hate* confrontations (ask my wife), and this strikes me as a situation when you need to buck up and talk to them.