When a collection account is opened against you, the collection agency by law has to notify you in writing and give you 30 days to contest the collection. Believe it or not, this is just as much for their protection as for yours. Collections agencies buy accounts from the original creditor (in your case, Cox) and this 30 day period protects them for trying to collect on accounts that have already been paid and/or are fraudulent.
Did you not receive the original letter informing you of the collection account? Did you not read where it informed you of your 30 day right to contest the account? Have there been no active attempts at collection?
Anyway, TCI (now ATTBI soon to be Comcast

) tried to do something similar to me roughly 3 years ago, except I didn't own the box. I moved and they insisted I return the cable box and that they would give me a new one at my new residence. I gave them the old one and took a receipt for it from the install tech. A month later, I get a bill for $480 bucks for not returning the box. Naturally, I was pissed. Thankfully, I kept the receipt for returning it and gleefully ripped some very rude TCI CSR's some new assholes and the matter was dropped.
Like Amused said, contest the collection, both to the bureaus and to the collections agency that purchased it. They have 30 days to prove that the collection is valid. Removal from your report may take up to 90 days to show. Hopefully, you kept your records. I would also file a complaint with the BBB and your state's attorney general.