I use Word for Mac OS. Over time I have created a few custom templates that are no longer relevant. I can't seem to delete them. Is it possible?

I am not sure where templates are stored by default on a Mac, but am confident it will be in the same location as the normal template. Look in your Options > Advanced at the bottom. You will probably find a button for file locations. Click on that button and find the location for user templates.
Templates in Microsoft Word

And if they are not in the User Templates, they have to be SOMEWHERE. If you know the names of the dot (or dotm) files, simple do a search for them, and then delete them.