rbenvでsystemを選択するには

実はとても単純。
$ rbenv global system
でおｋ。

参考サイト紹介

What do you get when you do

rbenv global system
ruby -v

or

rbenv local system
ruby -v

The special version name system tells rbenv to use the system Ruby (detected by searching your $PATH).
Refer section 3.1 at
https://github.com/sstephenson/rbenv
ubuntu - rbenv not including system ruby (1.8.7) in list of available ruby versions - Stack Overflow

ん？rbenv本家に載ってる？

rbenv global

Sets the global version of Ruby to be used in all shells by writing the version name to the ~/.rbenv/version file. This version can be overridden by an application-specific .ruby-version file, or by setting the RBENV_VERSION environment variable.

$ rbenv global 1.8.7-p352

The special version name system tells rbenv to use the system Ruby (detected by searching your $PATH).
GitHub - rbenv/rbenv: Groom your app’s Ruby environment

ホントだ。
単純にsystemを使えば良いのね。

実例（Lion/OS X 10.7.5）

$ rbenv versions
  1.9.3-p194
* 1.9.3-p392 (set by /Users/hoge/.rbenv/version)
  macruby-0.13
$ rbenv global system
$ rbenv versions
  1.9.3-p194
  1.9.3-p392
  macruby-0.13
$ ruby -v
ruby 1.8.7 (2012-02-08 patchlevel 358) [universal-darwin11.0]

アスタリスクの選択マークが無くなっちゃうのね。rbenvでインストールしたRubyじゃないから、ということでしょうか。