If it is every user and you don't want anyone at all to use another style then in the AdminCP, then you need to change 2 settings:
vBulletin Options ->
Style & Language Settings ->
Default Style to the style to be default.
vBulletin Options ->
Style & Language Settings ->
Allow Users To Change Styles to
No.
It is a all or none with standard settings.
If you want to fix it for some usergroups or members and allow others to choose freely then check out
My Hacks - I have force style hacks for v3.5.x, v3.6.x and v3.7.x versions that will do what you need.