Unless the municipal enforcement covers your yard (in all likelihood they do not), it's the land owner's problem to have offending cars fined or towed. Many apartment companies subcontract this to commercial parking enforcement ~~scams~~ companies, but they won't actually do much except stick "fines" (which legally speaking are not fines) on the windshield of offending cars. If a car has to be towed, they won't do it AFAIK.

So probably you would have to complain to the building administrator (isännöitsijä), who will likely proceed to not do anything about it until many other people complain too.

If that fails, mild vandalism is always effective. Let the air out of the tyres at night (sticking a green lentil in the valve cap works wonders) and leave them a friendly note calling them a moron and an asshole.