There's a way to do it in Unity (CCSM), but it won't work for Unity-2D.
i am in 12.10 and when i was tweaking the unity bar suddenly went away!!
restart does nothing
I had ccsm installed so through the terminal i launched ccsm in there i went to the desktop option and Ubutnu unity plugin was no there
another answer to another question told me to type unity --reset
This then relpied that
"the program 'unity' is currently not installed.
Unity is enabled by default in compiz, which makes it difficult to use compiz with another DE.
In fact, compiz was originally designed to be modular and can still be used with other DE.
To disable unity from a running compiz session (such as Unity itself), try this.
This method has the advantage of not having to do this inside a broken desktop environment.
0. Log in to a non-compiz DE