i think you're talking about the style. It switches sometimes while browsing/navigating.
go to style manager->choose Default Style->Edit Settings in dropdown->Allow user Selection->Set to NO. This would prevent any user to switch to default style. But admin would still be able to switch.
To disable even the admin switching of the style
go to vbulletin options->Style and Language Settings->Allow Users To Change Styles->Set to NO
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