I had a similar problem when I bought my cottage. It was being serviced by a large concern in Laconia and the seller informed me that the remaining propane was paid for and that it was now mine. I called the propane folks to transfer the account and was presented with a lengthy legal document to sign. After reading it, I decided that I wasn't about to sign-away all of my rights for three generations just to buy propane and told them so. Their response was to come out and disconnect the tanks. My response was to reconnect the tanks and continue to use-up my propane.
Once the tanks were empty, I moved them to the side of the driveway and notified the company to pick them up. [They suggested I get a truck and bring them in myself.] A year later, after repeatedly requesting the tanks' removal, I sent a simple email asking them if they were going to pick up the tanks or if they were abandoning them. If the latter, I said I would notify the authorities of the abandoned tanks and see if they wouldn't remove them for me.
The company picked-up the tanks the next day.
[BTW, I don't know what "authorities" are concerned with propane tank "dumping", but I'll bet there's at least one.
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