You could try different sized nut drivers. If you can find a size that "grips" the edges when lowered onto the head of the standoff, you can possibly use it to turn it, as if you were using a socketed wrench. This is a technique auto mechanics use.

One of those 'one size fits all' sockets might work. First time I'm ever recommending one of those.

i use to work in a wood shop that made chairs an tables and book ends. they have a special tool to use on these items..(so folks don't fix broke stuff) at least that's what the boss told me. So now you will have to make something at home. sorry for the heads up....

I figured this was some kind of security screw

Looks like a simple triangle spanner would work

You could try one of these:

https://www.irwin.com/tools/locking-tools/the-original-locking-wrenches

It bears at three points and is designed for dealing with really stuck-on flare-nuts on brake fittings and the like.

An alternative would be to take a Dremel to it and grind it into a regular hex nut then use a wrench on it.

Those are too big. Someone suggested a triangle wrench and i think that will work.