Once a Payment Gateway has been used, it cannot be deleted. The Gateway will need to be inactivated  which will remove it from use and as a feasible option for use later. The Gateway will need a password entered to be able to inactivate it.  

To inactivate the gateway:
  1. Go to Core > Settings > Payment Settings > Payment Processing
  2. Edit into the desired gateway and uncheck Active
  3. Make sure you have a password entered for the account
  4. Click Save
 After the account has been made inactive, the school should also get in touch with their payment gateway provider who can then remove the account from their side.