To hide default site templates available to the Template Picker when creating a new site, edit the WEBTEMP.XML file in the 12 Hive at web server extensions\12\TEMPLATE\1033\XML. Change the parameters for Hidden from No to Yes for all the templates you want to hide. Then restart IIS from the command-line to reload the configuration file. The templates will now be hidden. Any tabs that were available in the picker that do not have templates are also hidden.