Banks and companies love automatic / electronic payment, since it means they don't have to pay for people or equipment to handle paper checks, but once somebody has your numbers, it can be very difficult to get them to stop using them. The only sure way I know to stop a direct withdrawal is to close the account and put your money in a different bank. (If it's still at the same bank, "helpful" bank employees can easily switch the direct debit to the new account.)
It's really strange that the bank won't let you cancel the DD "because you're in a contract." Maybe you are (see below) but that's none of the bank's business. It's the same as anybody else you might owe money to: if you don't want to pay for some reason (maybe the person didn't deliver the order or do the work) then you just don't write the check. The person can't go down to the bank and tell them "he really owes me money, so why don't you just give it to me?" Even if you legitimately owe the money, and the person is going to sue you to get it, the bank shouldn't be deciding when you're going to pay.
The other problem for you is that, as part of making the "one off payment," you may have gotten into a contract with the company. That is, you may have agreed to pay for whatever it is, for your ex-partner, for six months or a year or whatever. If that's the case, then if you stop paying, they may be able to sue you for breach of contract, refer the account to collections, etc. You should figure out what the company thinks your relationship is, and what will happen if you stop paying. (Remember, you may have to close your bank account to stop!)
You might be able to sue your ex-partner for the extra payments you didn't agree to make, but it's a hassle to do it yourself, and it's expensive to hire a lawyer. If you can just stop future payments, you may want to call it good.
You will likely need to close your account and open a new one to get the electronic transfers to stop. You could also use the stop payment procedure at your bank, but there is no sure way to stop electronic debits unless the account is closed.
In the future NEVER authorize anyone to access your bank account. NEVER give a creditor permission to access your bank account, not even once. Also, never keep your money in a bank where you owe money.