'revoking' a userdata

[DC <dcharno@...>]

Is it possible for the c/c++ side to 'revoke' a userdata from a script? 
 For example, an object which exists in one level of a game, but is no 
longer used or valid in the next level?  If the script forgets to 
release the object it could persist.

One solution has been to destroy the lua state when transitioning 
between levels.  But, I want to be able to pass and maintain state.