Hi Chrissa
Actually, there's a free and more permanent way of doing this:
Go to this link :
http://www.consumeractiongroup.co.uk/forum/let ter-templates/131250-dca-creditor-harassment-t elephone.html
If these people are ringing you constantly, then they could be in breach of Section 40 of the Administration of Justice Act 1970, and the Protection from Harassment Act 1997.
They know this and one letter from you will stop them.
They've probably had the debt offered to them, (but not assigned - this is crucial) by the original creditor. Basically they only get paid if they retrieve payment from you which is why they're so dogged in their chasing.
It's harassment and they will stop instantly if you write the simple letter (template in link above).
Good luck and let us know how you get on!
Best wishes
221b