Worst service thus far

It has the nightmare dealing with bank so far, I have been trying to close my credit card with them but they keep making it so difficult for me to do.i settled the account I'm March but I keep getting calls from collections that I still have an outstanding amount and I agreed to pay it and today the amount has accumulated interest how I don't know and as far as I know I had asked for the account to be closed and was sent a settlement letter but I'm still charged interest for what I don't know.This is worst bank bank ever service is messed up. Their consultants need to be trained effectively so then can better handle situations .