New way
Use the uninstall command: rbenv uninstall [-f|--force] <version>
rbenv uninstall 2.1.0 # Uninstall Ruby 2.1.0
Use rbenv versions to see which versions you have installed.
Old way
To remove a Ruby version from rbenv, delete the corresponding directory in ~/.rbenv/versions. E.g.
rm -rf ~/.rbenv/versions/1.9.3-p0
Run rbenv rehash afterwards to clean up any stale shimmed binaries from the removed version.