No, you can't edit Font Book collections

After vigorous searching for "how to delete a font from a group in fontbook" and "how to remove a font from a collection in Font Book" and such, I've concluded that there is no such feature.

These are not the options you're looking for.  Move along.

Did you misfile a useless symbol font in your "Stencils" folder?  Well, valued Mac user, you'll just have to deal with the consequences of your actions.  Apparently you used to be able to drag the offending font onto the desktop and letting it go poof.  However, that's definitely not working for me on 10.11 El Cap.

There's a simple workaround:  The collections themselves can be deleted.  Rename the contaminated collection and create an empty one with the desired name.  Move all the stuff you actually want into the new collection, then delete the renamed contaminated collection via the File menu.

I fully acknowledge that this is going to be useful to precisely nobody I know. Maybe Hugh.  I'm just posting it in the vain hope that it will short-circuit the next person's fruitless search for a nonexisting feature.