VERY ANGRY AND ANNOYED

I am so sick and tired of these people, i had an account with them and it was paid in April, 13th of April but they keep on calling me , I told them that the account was paid in full, they ask me to forward the POP off which I did, but still I received call demanding payment,the payment is still reflecting on ITC, how do they close month end ? how do they allocate funds that reflect in their bank account, because they asking you to use your ID for ref. but that is not helping, I AM SO ANNOYED AND IRRITATED NOW,