First thing is to get back control of your site.
Assuming you have FTP access, you can access and edit the config.php file. In it you should see:
Code:
$config['superAdmins'] = 'X';
where X is a user profile number (by default user #1). Change that number to your user ID and remove any other numbers. That will make you a super administrator and remove anyone else from being a super administrator that was listed there.
You will now be able to log into the ACP. You can remove this admin and ban him. Also remove anyone else who is a super administrator. You, the site owner, should only ever be a super administrator. Anyone else should only ever be administrators. Only super administrators can add or remove other administrators.
As for your domain name, unless you can negotiate a transfer from this person, which may be unlikely, you have lost it and he could point it to another website, so yours would effectively disappear. Your only solution if he won't transfer it is for you to buy another domain and then point that at your current XenForo installation. You will also have to update your domain in the XenForo customer area.