FYI - a "superuser" is technically a system administrator that has what is called "root" priviledges. Any computer system has one. Therefore, any on-line poker site has superusers. Quite literally, they are God to the system.
The key is the trust that you have in these types of people. Technically, if you take an on-line gaming site, the administrators should be prohibited from playing on it, be paid enough that they won't have to look for money elsewhere, and go through a thorough background and personality assessment prior to granting them root access.
These types of problems are not limited to on-line gaming sites. Even something like Amazon has to worry about compromising their customer's personal information and prevent the system from superuser fraud.