How can I disable the keyboard Eject button ?

You can assign another eject key to another key, but you still can't disable the original with Karabiner Elements.
Low-tech, but my solution works great!

This has finally bugged me to the point where I'm trying to find this same answer. It looks like this question has been asked for 10 years without a good answer (based on some quick Google searching). Admittedly this is a recent annoyance because I've invested in a sit/stand desk that I'm finally using at ergonomically correct heights. The right position is lower than I'm used to and my pinky keeps hitting that **** eject key instead of the delete key.

What's driving me most nuts about it is my Mac Pro's drive cover gets stuck in the down position on occasion and I have to fix that too. In my 10+ years with Apple this is the first annoyance I have not been able to find a software workaround for. I love the Apple keyboard, but I may have to go to a third party to get around this.

KeyRemap - Now Karabiner - https://pqrs.org/osx/karabiner/

This app lets you fully customize the keyboard and designate specific functions to rarely used keys.

As of 8/29/17, Karibiner does not work with MacOS Sierra but the app's site claims they are working on it.