Stripping single name/value pair from QUERY_STRING

I want to strip a single name/value pair from a QUERY_STRING when
Googlebot visits, and redirect it to exactly the same URL...only without
that name/value pair at the end. Is this possible using mod rewrite
alone, or will I have to do it at PHP level instead?

***
To elaborate a little more, Googlebot has picked up a PHPSESSID for
hundreds of pages. Or rather, it's picked up multiple PHPSESSIDs for
each of these pages, which is not only a ridiculous bandwidth hog but is
affecting search ranking too (it sees them as duplicate pages).

I've since modified the PHP session code to prevent Googlebot picking up
PHPSESSIDs when it visits, but unfortunately it's already got hundreds
of URLs stored with PHPSESSID name/values in its database, so it won't
drop them because they always resolve. So I need to do what I said in
the first paragraph. Strip it out and do a permanent redirect, bearing
in mind the number of other name/value pairs in the QUERY_STRING is
unpredictable and must remain untouched.

TIA
--
John