Paid but still being harassed via sms

Paid amount as stated in sms received on 29/06/15 on the 03/07/15, received a thank you sms 04/07/15,now still receiving sms's stating I must make arrangements for a debit order. How is this possible, I have paid this amount how come I am still being harassed? There is no outstanding balance, on querying this was told I now have to pay an additional amount to pay their fees? Totally disgusted with these people!