Register a trademark in your state for the same company name ;)
Also, send out a few invoices with that business name on it, that way you can prove you have a legitimate interest in the domain if they do get to the lawsuit stage ;)
You will also be able to contact them once you have the business name reistered in your state by telling them you are closing the business and that you want to liquidate the assets including the domain and how much do they wanna pay you for it
We have done this several times as a company and trust me, it works
REALLY well ;)
Regards,
Lee