Making a Form Template inactive will not delete the template but remove it from the Active Template list to an archived / inactive list, so that you are able to reactive it should you need it again. This function is only available within the Web Portal and not available within the App.
To Deactivate a Form Template in your Web Portal go to Forms > View Form Templates > Select the checkbox for Template/s you would like to deactivate > Select Deactivate from the pop up menu.