To inactivate the gateway on the onMessage:
  1. onMessage
  2. Settings
  3. Event Registration or Giving 
  4. Payment Processing
  5. Edit into the desired gateway and uncheck Active
  6. Make sure you have a password entered for the account
  7. 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.