wont stop harrasing me

Ive got MDC that wont stop harassing me for an account that is over 7 yrs old I've told them to stop phoning me and they wont I've told them I'm not paying this account it was R600.00 7 yrs ago in Nov they told me it was R6 000.00 now yesterday they want R13 000.00 from me i have never received a statement just this sms's every 2 weeks