SAP delivers different skins/themes for the WebUi which can be changed by the Users in the personalize Section.
If you want to hide skins...
Goto SM30 -> Table = CRMC_THTMLB_SKNE -> click on maintain -> click on change mode -> select the skin you want to hide or de-activate -> save
( Taken from the original Post: https://www.sdn.sap.com/irj/sdn/thread?threadID=1113990&tstart=0 )
In our CRM 2007 System you have to add the skin you want to disable. (Table was empty on first access)