Collecting on Prescribed Accounts

I'm being constantly harassed by this company on a loan that I had with FNB which I know is prescribed. My last payment was in 2013 and they did not contact me to make payments arrangements for three years. This year I started receiving calls from Norman Bisset and I informed them that the loan was prescribed.

Suddenly this month Groenwald and Lubbe are calling me. I explained to the agent this morning that the loan is prescribed and he proceeded to send me a threatening SMS.

Why are they allowed to collect on prescribed accounts???