Is there a way to deactivate the user's ability to deactivate GuiXT from within SAP?

This is possible with Sapgui 4.6D (Patch 103) via a registry entry in 
HKEY_CURRENT_USER/Software/SAP/SAPGUI Front/SAP Frontend Server/Administration  
Add a string variable with the name "DisappearItems" and value "Activate GuiXT". If this variable exists already with some other menu entries, you can add the new entry "Activate GuiXT" at the end separated by ';', e.g. "Generate graphic;Activate GuiXT".

The user will then no longer be able to activate or to deactivate GuiXT.  If GuiXT should always be active, you have to set the variable "GuiXT" in   
HKEY_CURRENT_USER/Software/SAP/SAPGUI Front/SAP Frontend Server/Customize 
to 1.