How do I cancel the payment plan? I don't owe as much as I thought, and I'd like to uncheck that box so that I can pay what I owe directly out of my bank account.

If you have not submitted your return for e-file, or printed and mailed it, you can return to the File section and select a different option for payment.

If you have submitted or mailed in your tax return, you can't change the payment option,  but you can still pay the amount in full. If there is no balance due, there is no payment plan. 

It isn't giving me the option to change my mind about a payment plan. I don't have any checks, so the best option for me to pay what I owe to the state is by a direct debit from my bank account. It won't let me because I originally chose  a payment schedule. I need help undoing this. 

You can go to the IRS Direct Pay website and just pay in full.  If you do this your payment plan will automatically be cancelled.