Hi Sebastian,
ArcSDE is designed so that users inherit permissions through the PUBLIC role to access the ArcSDE Repository tables. This is necessary for a geodatabase user to function successfully. It is possible to revoke those permissions from the PUBLIC role. However, this is not how ArcSDE is designed to work. Esri is aware of the security concerns this causes users. The following enhancement request has been submitted: [#NIM010483 EnhReq: Security concerns with PUBLIC's privileges automatically given to metadata tables].
Regards,
Melanie W.