Most of the default templates that come with Scrivener are not applicable to me - I’d like to remove them (or move them out for now) so that when I run it, I only get choices of the very small number of templates I actually use. I have searched on the disk, including hidden files, but can’t find these anywhere - is it possible to find them and remove them? I had assumed they’d be just files in a Library support folder somewhere but I can’t see any evidence of them.
It is not possible to permanently remove them, but you could temporarily (until you re-install or update the software). They are stored in the Scrivener.app package itself, under the Contents/Resources/Default Support Folder/ProjectTemplates sub-folder. You could try moving those out to an external folder somewhere and restarting the software. I make absolutely no assurances that this won’t cause instability however, use this trick at your own risk. 
I’d keep a copy of the original download around for safe-keeping in case you need to restore.