1 thing - do not do anything which gives them a legal leg to stand on.....and document / diarise all your efforts to get paid..
U choose -
1) personal service approach....
but way less stress is 3rd party
2) Legal service approach.... (Before U do this check how much equity they have in the site as U would aim for full cost recovery including legals - if v.low refer below re-mortgager).
(Debt recovery often means profit loss using agency due to costs - at least legal approach cost get awarded in this sort of case.)
Before U go too much further - get a court order over the property to ban sale / transfer of ownership if it is not clear.
U can also find out whether property is mortgaged (assuming it is a house / domestic property / office construct) and approach lender as they have a vested interest in the value of the property remaining intact.
Best of with good & speedy resolution.
Cheers
Rick