You can delete a template if it has not been used to make a form (or action or memo) that has been Saved to Project. In that case an administrator can use the web app menu option:
Admin => Templates, select a template, then use the button with 3 dots to select Delete. If the template is not delete-able then the 'Delete' option will not appear.
Otherwise, an administrator can make the template inactive as follows:
Use the web app menu option Admin => Templates, select a template, then Edit, then go into Advanced Settings and turn Active Off then Save.
Alternatively, you can use make a template no-longer available in projects via Default Templates or Project Templates.