Breach of Contract altering given mandate

Despite receiving excellent service from this institution initially, it went pear shaped. They decided to raise a debit any time there was money in my account, this was when I was not in arrears at all and payment is due on 1st of every month.

They were emailed and advised, this is in breach of our contract and my mandate for 1st of month.

I warned them if this happen again, I will stop paying them.

I have to pay monies to embassies for a client doing exports and they have taken this money.

When this happened again, I stopped paying them and all their collecting agents were informed why I stopped paying them.