I agree with Christina and everyone else on this posting. Once you contact the collection agency and inform them that you do not want them to contact you any more they are only allowed to contact you under certain situations...however for this to be valid you must send them a written confirmation of your request. If it is the Original creditor they do not have to abide by the FDCPA, but if they sold your debt to any third party then they are bound to follow the FDCPA (Fair Debt Collections Practices Act).
Make sure its not the OC first, if its not send them a letter CM so you have proof that you sent it, you can easily google C & D letters for Collection agencies to get a general idea of what the letter must contain. After you do all of that then if they contact you again you have claim to bring suit. You can either lodge a complaint with your State AG office, which really is the easiest way, or you can bring a claim to small claims court. More often than not it will be a Small Claims (if you read the FDCPA each violation is $1000) but that becomes a pain.
I am not giving you legal advice, I use to work for a Commercial Collection agency and that entailed learning the right and wrong ways to go about debt collection, most times a lot of collection agencies have you on a automated call log, and only when you pick up will it be transferred to a collection agent, so a letter will bring it to their attention. However like Christina said, depending on the dollar amount you owe, that will sometimes motivate a collection agency to file suit against you.
Hope this info helps