Manual:$wgSkipSkin

Details
Specify the name of a skin that should not be presented in the list of available skins. Use for blacklisting a skin which you do not want to remove from the  directory

Example, in LocalSettings.php put: Above would remove all skin choices but monobook. BTW, "standard" is the skin called Classic. Omit from above array any choices that you want to remain. See also: $wgSkipSkins