If you recently received notice of your credit limit and the rest of the account-opening disclosures, you may be able to cancel the balance transfer. The bank is required under 12 CFR 1026 Truth in Lending (Regulation Z) to give you a chance to decline the balance transfer once you learn your credit limit and receive the rest of the account-opening disclosures.
Generally, you have at least 10 days after the bank sent the account-opening disclosures (not the day you received them) to call the bank and stop the balance transfer.
If you did not contact the bank in time and the balance transfer was made, you will need to pay off the entire account balance. Once you have paid the account in full, you can cancel the card.