How do you delete custom templates from Pages Template Chooser?
The custom templates are stored in Finder in:
yourusername*/Library/Application Support/iWork/Pages/Templates/My Templates
If you are using OSX 10.7 or 10.8 the System folder is hidden from you by default, show it by option clicking on the Go menu in Finder.
When you locate the template you want to remove just drag it to the Trash
* The one with the house icon.