Just delete /System/Library/Frameworks/Ruby.framework, /Library/Ruby, and erb, gem, irb, rdoc, ri, ruby, and testrb from /usr/bin.
Warning: OS X expects and sometimes relies on frameworks and software shipped with the system. It would be a very bad idea to remove the system Ruby. It might even get replaced next time you update OS X -- or it might not, but OS X might expect it to be there, and then bad things will happen. Remove it at your own risk. I am providing this answer because you asked, but I would not recommend removing a system framework.
Why do you want to remove it? It's not hurting anything to sit there, unused.
find /usr -type f -name \*.rb. That will be one thing that is affected. Applications could expect ruby in/usr/binalso, and they will fail. In general, it's not a good idea to mess with executables in/usr/bin,/sbinor/usr/sbin. – Greg Nov 22 '11 at 23:32.rbextension? – Daniel Beck♦ Dec 5 '11 at 18:53