You would need to change their member level first and then delete them or just keep them as a default member level after changing it. If you think they'll need to log in again at some point. If so, you could then change them back to super admin.

Always a good idea to have more than one super admin users for yourself too. The key for security is also to keep the super admin users just for administrating the site and have another user for yourself for posting, friends, interacting, etc.