Your account number:
With regards to the above mentioned debt you allege I owe, I dispute owing this debt. If your client believes that I owe them money, I wish to have them take me to court and prove it before a judge.
In the meanwhile, you may not contact me with regards to this debt.
Sincerely,
The problem with many notes of this nature is that if you acknowledge the existence of the debt... or if you acknowledge ever having given them money on the debt... even if that acknowledgement is accompanied by a refusal to pay the debt... you have effectively acknowledged the debt and reset the limitations period. You do not want to do that, which is why an explanation of your position should not be included in the letter. If necessary, the explanation can be made before a judge.
Collection agencies are notorious for attempting to get a default judgement without actually serving you papers. However, if they do this, simply make an application before the courts to have it set aside for lack of notice about the proceeding.