Just close your helpdesk and you won't be charged anymore. If you close your helpdesk it will stop the billing and you will not lose any data. Please note: Closing the helpdesk will stop the functionality of the helpdesk, but you will still be able to login to access your data and/or upgrade to a paying plan at any time.
To close the account in Mojo Helpdesk version 1 go to Manage> All Settings> Account Information> Close Helpdesk.
To close the account in Mojo Helpdesk version 2 go to Helpdesk Settings> Close Account (in the Account Information section).