If you need to remove a license from one of your sites, go to Settings > iThemes Licensing.
Select the products you'd like to remove the license from and then enter your iThemes username and password below the list of plugins. If needed, you can reset your password here.
Click the Remove Licenses button and the license will be removed.
If you are trying to remove someone else's license from your site (for instance, a developer installed the plugin for you but you now have your own iThemes account), using your iThemes credentials will still remove the license. You do not need the previous user's information.