This seems like it should be easy

I'm simply trying to remove the handle from the fuel tank L - R - OFF selector. I'm relatively sure this is the standard finish kit selector. It has a screw through the top to hold it on, but even after removing that screw the handle is still tight on the post of the selector. I'm reluctant to apply a whole lot of force, the fear being that I might break something in the selector.

Is there anything else (other than pure cussedness) holding this handle on??

Swear at it. It always works for me!

Might try a drop of penetrating oil. The screw is the only thing holding the handle on. I think the handle is cast but the spindle it is attached to is brass. A small gear puller, if you have one, might also work.

Here's what worked for me. Put the screw in all the way and then back it off a turn or two. While pulling up firm and straight on the selector, tap the screw with a small hammer. If this doesn't work, I'd go find a small gear puller. The shaft has a slight taper so the knob can get stuck pretty good.