Hi We have a mortgage with AMLAK with a high rate...

Hi We have a mortgage with AMLAK with a high rate (7.5%), our agreement is on the base of rent. The rent has dropped a lot but unfortunately they didn’t decrease it, because they have a direct debit form which has signed by my wife and they are taking the money as they wish, please let me know how can we stop the direct debit and make a complaint against them?