Start by looking up your city code to see if there is an ordinance prohibiting unregistered or inoperable vehicles on private property. If there is you can give them proper written notice that their tenancy will be terminated if they don't get rid of the cars or get them registered.

The following website has links to all the statutes addressing landlord-tenant relationships. Study up.

https://www.landlordology.com/massac...d-tenant-laws/