Keep in mind that you can only delete forms that have no related payments and/or plans attached to them. All other forms are permanent so that both you and MoonClerk have records of all payments made on your account.
1) Click on the Forms button in your account's main navigation
2) Click on the form you'd like to delete
3) Click on the Edit button
4) Click on the "Delete this form" link below the "Update Form" button
5) Click OK to confirm that you'd like to delete the form