Willpro, you seem to be having a very thorough look at Gibbon, which is great to see. At current there is no way to prevent access to any of the preferences functionality, except for Personal Background. This feature can be disabled at Admin > User Admin in the main menu, and then Manage User Settings in the module menu.
If you want to stop users swapping themes, you could look at having only one theme installed. Hope this helps.
Willpro, thanks for the positive feedback. Great to head you are enjoying Gibbon!