Suspend a one-time debit
It is possible to oppose a direct debit if there is a dispute concerning the amount of the invoice. In this case, you must:
send (in acknowledgment of receipt) a letter of dispute to the creditor, explaining why the invoice is disputed, and that the levy will be blocked;
send (in acknowledgment of receipt) a letter of opposition to your bank, indicating with more precision the payment that you refuse (name and address of creditor, amount and date of collection).
According to the contract you have with your bank, this opposition will you be charged, but it is often possible to negotiate with your customer advisor for this is not the case.
If the bank passes the levy despite opposition (which is possible due to delays in processing or automation of collection procedures), you can ask to be re-credited with the amount debited, at no cost.
Stop a Debit Permanently
To permanently stop a direct debit, just send either a fax or a mail to the bank with:
name and bank account information accurate,
clear designation of the debit in question (name of the organization concerned, deduction amount if it is constant, date of collection if it is constant),
the request to cancel the direct debit,
the date on which such cancellation to take effect.
Cancel a levy does not settle his account with the organization who benefited. It may be necessary to pay the balance of the account separately, including by check. But that may control the relationship, especially when breaking a subscription.