In the older builds of ServiceDesk, we did not have the function to enable/disable Incident templates. In order to disable an Incident template, you have to delete the particular template from the incident template list.
If you have any existing tickets created with the particular template, then deleting the template will move it to be in a grayed-out/disabled state and will be marked as "Template, not for further usage" . If the template is fresh without any ticket associations, then on deleting, it will be permanently removed from ServiceDesk. In the future, if you need this template to enabled you can uncheck "Template not for further usage" option which will turn on the template to an Active state.
In the newer build ( 10503 ) of ServiceDesk, we have enhanced this feature ( SDF-57857). Now the technician can enable/disable required templates from the incident template list view.
Click here to download the latest Service packs.