It seems the nature of the T&T forum to morph mundane, everyday situations, or scenarios that are more than likely completely banal into the sinister and potentially, if not overtly, hazardous.
People see a post here and, quite naturally, the brain switches into how best use my gun to defend myself in this scenario.
However, I feel that it would serve everyone best when, on the face of it, the situation is not an open threat to remember that fact first and foremost.
In the OP situation, as has been described, the only aspect that points to this being a credible threat is by association to it being posted in T&T, not the details of the situation itself.
By all means allow the gun to play a passive, background role in your thinking (eg, have it nearby at home if you've seen such a "suspicious" car earlier) but let normal behaviour rule the active part.
Don't feel good someone parking across the road? (perfectly understandable, BTW.)
Call the police, although they may well as what it is exactly that the occupants are doing wrong. Otherwise, floodlights seem to work. Passive, but effective.
You can even pick-up a nature-watcher's N-V monocular and watch from a darkened room, if needed, to assure yourself that they are not observing houses.
If it is an amourous couple, on the hand, try to have the self-discipline to put the monocular away...
Seriously, though-
Just because it is a thread in the T&T subforum of TFL gun forums doesn't mean that a gun-related response is the best advice. Let's look at the proposed situation, for what it is, first and then go from there...
