A colleague asked me about removing some of the templates/site definitions that are available when creating SharePoint sites. Actually It’s so simple.
To hide a template from users :

1) Open the Webtemp.xml file located in the the 12 hive /template/1033/XML folder.
2) Change the Hidden parameter of the template(s) you want to hide.
3) Run IISRESET

Note also that you can create your own categories by editing the DisplayCategory value