Under Options > Controls > Keyboard you get your typical list of stuff that happens when you press a button.
For example, "J" Jettisons my Cargo, etc.
Problem is, I've got a bunch of crap assigned to my 'ol gamepad joystick by default (?) and I wanna clear all of it out.

The help says:
Mouse over a control and press "backspace" or "delete" to delete all keys bound to it.

I've tried that. Really I have. I mean, I am really good at pressing delete and backspace. I hover over the function that I want to delete all keypresses for, and press delete. Nothing happens. I press Backspace. Nothing happens. I use harsh language. Nothing happens.

Am I missing something utterly basic here? No, really, I admit I might be missing something quite simple.

How can I clear all keybinds for, say "Accelerate" and assign only one key character or joystick button to it?

I'm gonna go delete my WhoGivesAFuck.cfg file and see what happens.

LeberMac!

Yo Leber! Yes, the GUI to delete binds is kind of funky. I do seem to be the only person who can get it to work! ;)

Anywho, if you bind a key or button to NONE, it will clear that bind. So /bind JOYBUTTON1 NONE will clear the binds for button 1. A way to clear all binds for a specific command would actually be nice!

Hm, yeah, something seems to not be working properly there. Thanks for the report.

When mousing over each row, it should turn yellow, then you can press Backspace to clear it, but that doesn't seem to be working.

Indeed, it does not seem to be working.

Look at the bright side, now you have something to fix! I have given you purpose.
You're welcome! ;)