Shadowing a session can be prevented through:
Elimination of the option through User Manager
Override of the settings in Citrix Connection Configuration
User has higher color resolution
User has higher screen resolution
User is at a console session
Shadower is at a console session (unless shadower of user is at an ICA session at the console)
User/ shadower is on an RDP session

If the administrator is at the console nothing changes, shadowing is governed by the user and shadower.

Reference: MetaFrame Administration Module 9
Reference: MetaFrame Books Online- Search for: Shadow