How can I disable the "Register domain" link on my Plesk 8 Linux server ontrol panel. I do not want my customers to be able to register domain on their control panel through this link with Plesk Registration service.

I had the same issue couple of days ago... you can do the following as admin user:

Server -> Interface Management -> Interface Templates -> Preferences

There you have to check all the boxes in the "Buton sets" section.

Regards,