Hi papa T,
I doubt that a 'bad template' is the cause of this strange issue.
Leaving that aside, here is the path to deleting templates that you have created and saved:
Close Numbers. Go to the Finder.
In the Finder, press the option key, go to the Go menu and choose Library.
This puts you in your user Library—note that OS X maintains several Libraries, one for itself and one for each user account on the computer.
From your Library, open:
Application Support > iWork > Numbers > Templates > My Templates.
In My Templates, select one or more templates, then press command-delete to move the selected templates to the Trash.
Empty the Trash to permanently delete these templates (and anything else that has been moved to the Trash).