Wilky,
The reason concrete blocks / tractors / tugs etc are placed in front of the aircraft is to stop the risk of them being moved to another area of the airport where a new lien could be placed on it. i.e., someone's hangar or parking area where in an instant the lawyers could be waiting to put in another claim. Mechanics liens etc. can hold up the whole repossession process for some time, costing the aircraft's actual owners several hundreds of thousand dollars a month.
Hope that makes some sense