Prevent serialization of an environment?

Within I'm using an environment as a hashtable to cache the results of some
repeated computations in a package. The thing is that the contents of the
cache will not be valid across R sessions, so I would like to make sure
that the cache is cleared if the user saves or restores the workspace (or
when a package is prepared for lazy loading, etc.). Is there a way to mark
an environment so that its contents do not serialize?