Unless there is some mechanism by which residents of the other strata plan have agreed to abide by the by-laws of your strata plan then there is not much you can do (worth checking, though). It’s the same as someone from a house down the street getting access to your parking.
However, I would leave a note on the car saying something like: “This car is not a bona fide visitor’s vehicle nor does it belong to a resident of this strata plan. By leaving it here you are trespassing. If it is found in this space again it may be treated as an abandoned vehicle and towed onto the public highway where it will be removed by police.”
Now, threatening to that is one thing but actually doing it is probably illegal. However, I can’t honestly say I wouldn’t do that if I were in your position.