Should I send a C&D letter?

I'm almost out of debt! ....except for student loans. I have a couple of REALLY old way past the SOL accounts. They are both with Portfolio Recovery. I have recently received letters in the mail from them and both stated that due to the age of the debt they will not sue me. I get a call once in a while from them but, they do not leave messages. Should I send them a C&D letter? Would that prevent them from sending letters as well as calls (to me or anyone else)? Also, could someone please direct me to the proper format for such a letter that would not restart the SOL? Although I could throw some money their way, it would be better for me to use that additional money to pay down the student loans. Thank you for any help!