I'd go for a friendly note under the windscreen wiper just pointing out that it's your property and you'd appreciate if they'd park in the visitor's spaces, or wherever. If they still persist, I'd install some metal poles (while their car is in place if necessary
).
We have this problem with our allocated space in a car park round the corner from our house. Sadly our spaces are unmarked so when I get round to it, I'll be painting a space and putting a number in it. I'm not really bothered as we never use it, but as mentioned above, it's your property and you've paid for it!
Chris