RCN Cable Co. Charging me $589 that is not owed to them by me Internet

I have tried numerous times to communicate with these people to no avail.  I did own a condo at (((REDACTED))) Sheridan Road for 13 years.  I did sell it effective 8/24/17.  This building is a condominium and the Board of Directors signed an agreement for RCN to furnish cable services for the entire building.  I signed no such agreement, and was never told the entire terms of the agreement. 

After the unit was sold and I had moved to my new home, I was told suddenly that I had to return the cable equipment to RCN or I would be charged something.  I left the equipment at the condo so the new owner could use it.  I am also a senior citizen.

RCN told me that they brought new equipment with a new serial number for the new owner. 

Despite phone calls and attempts to e-mail my problem to them (all e-mails get returned as not possible to deliver) I am getting a bill for $589.00 because I did not return the equipment (of which I was never advised by the Board of Directors or the Property Manager) and since the Board of Directors signed the agreement, it is their responsibility to be sure the equipment is returned.

It's not the responsibility of the unit owner, who made no formal agreement to return the equipment.

They continue to bill me and say they will put it on my records as an unpaid bill if I dont't pay it.

I don't owe this and will not pay it, but I never want it on my record as a collectible debt.  This is fraud, and easy income for RCN.