How can I uninstall Ruby on Rails on Mac OS X?

Are you uninstalling Ruby on Rails, or Ruby AND Ruby on Rails?

If you want to uninstall Ruby on Rails it should be a simple matter of using gem uninstall rails or doing gem dependency rails and then gem uninstall for each of the listed gems that Rails depends on. If you installed them system-wide, using sudo originally, you'll have to use sudo again to uninstall them.

If you want to uninstall a system-wide Ruby you installed from source, well, I'd probably leave it alone and install any new versions of Ruby using RVM. The older version won't be visible to apps unless you want them to see it or set your path to include it. RVM's version will be found first unless you do rvm system.

Actually you could simply install RVM and have it install your new versions of Ruby, then from there install new versions of Rails, without even bothering to uninstall the old stuff. RVM really is that cool.

Download RVM or RBENV and upgrade to the latest Ruby (2.3.0 at this point). Don't touch your system Ruby.