Iemas refuse to do debit order in one of my products with them.

I bought a car with Iemas last year. On the time of application I never got to be told that i have a choice between salary deduction and debit order of which I have signed them both. And since then they have been deducting directly from my salary up until i asked from payroll last week if this is a must? Only to find out that i have a choice and i didn't know that because i was never advised. I approached Iemas last week and they advised me that I cannot change it. My question is why did i sign the debit order form on the application and also i have other products with them that are debit order deduction since last year and i want to combine everything to debit order as this is affecting me in a lot of ways.