Can't find a way to delete Templates from the "My Templates" window in Pages.

If you want to delete user-made templates, go to ~/Library/Application Support/iWork/Pages/Templates and you'll find the individual files that you can just delete.

If you want to delete iWork's own templates, that's a bit trickier. Right-click (or Control-Click) on the Pages application, choose Show Contents. Drill down to Resources and Templates and there they are. I'm not sure I'd delete anything from the application package, however, so my advice would be to copy the app and try it on that to make sure it doesn't foul anything else up.

Good luck.