Depends who’s responsible for overseeing the parking for your building. It might be a private company if you have separate parking spaces owned by the building or tenants, in which case you report to that company. If not, then the city:

https://www.hel.fi/en/urban-environment-and-traffic/parking/parking-control

That’s managed by the city. They seem to have online form and a phone number for this kind of thing.

You can take a photo but as far as I know it’s not really that useful since (as far as I know) photos from individuals can’t really be used to give tickets.

Damn, when I lived in Helsinki, I got parking ticket for parking properly near my apt during the 5 min it took me to run home and send an email registering my plate for a list of tenant cars. Never thought someone would be asking this.

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.