A feature called Signature collections enables you to assign a specific signature to a user regardless of the one inherited from his group. Or even better, you can utilize this to let users choose from a curated list of signature templates.
You can do so if you click Users' details in the User menu and scroll way down. Here can you see currently assigned signature template and if it's inherited from a Group or not. If you wish to change it, simly click on "Change assignments" and pick the template of your choice.
This option also works for marketing templates.