Learned from my own mistake: Next time, it will be in the contract that it is the landlord's responsibility to have the designated areas clear on designated dates.
I wouldn't put the specific dates in the contract, but something along the language of: "Client and Contractor will arrange mutually agreeable dates and times for specific parking areas to be cleared. Once these times and dates have been set, the Client will be responsible for assuring vehicles are not parked in designated areas. A $xxx fee will be assessed in addition to contracted charges if designated areas are not cleared as agreed."
A lot of appratment complexes have agreements with towing companies anyhow to keep unauthorized cars out. If they post it well, and folks are still parked there, they can just have the towing company make room and let the tennenant figure it out with the towing company.