I doubt that it is illegal, after all they are illegally parking on your property! You just have to be sure that the owner can’t say that you maliciously damaged their property which WOULD be illegal, even though on your premises. It will certainly get their attention and then they have to remove it before they can drive off which may discourage them from doing it again.
You might also want to circularise your residents and tenants and advise them to be on the lookout for other cars which tailgate them into the private parking area. Of course if they let the other cars do it (for a mate etc) then they would be in breach of the by-laws themselves I would think.
It is really difficult to do any thing about this sort of thing as it is not a police matter as such. You could take action for trespass if the offender is a repeat one and you do have evidence by way of the CCTV footage. Active discouragement is the way to go. (Maybe you could talk to the other person on this forum who had a car mysteriously removed, plates taken off and trashed!)
Maybe your sign should also say words to the effect…
“your vehicle is illegally parked and is therefore trespassing on our property. We have photographic evidence which will identify you and action will be taken in any future cases without further notice as well as notification to the police”. Of course, don’t threaten too loudly unless you intend to take action.