In the latest build of Service Desk Plus 11006, we have introduced a function to filter Service templates. There are 3 filters "All templates", "Active Templates" and "Inactive Templates".
The Inactive template filter will show the list of templates, which are disabled. A Service template can be instantly Enabled/Disabled, in order to disable a template, goto [ Admin tab -> Service catalog ] you can disable the desired template from the service template list using the "Disable Service" button.