After a you make a payment group inactive, you can't add payment types to it or accept payments from its payment types.
Warning! Before you make a payment group inactive, make sure any POS sessions with that payment group are cashed up. You won't be able to cash up the sessions if the payment group is inactive. If you make the payment group inactive and that prevents you from cashing up a POS session, you'll need to make the payment group active, cash up the session, then make it inactive again.
Note: You can't make a payment group inactive unless all of the payment types in it are also inactive. That means there's no risk of you or anyone else accidentally taking away access to your site's payment types.