Previous owners' propane company won't get their tanks - are they mine now?

I started my own contract with a different company when I moved in. New company installed their own tanks. Old company (over the phone) said they would come get their tanks, which are disconnected and sitting on my yard. It's been months. At what point can I get rid of them myself? Or do I need to write a letter? I've never had a contract of any kind with the old company, nor received a fuel delivery. Previous owners thought I should just use up the propane but I wanted everything with one company asap and didn't think the old company would just...not get their crap.