[exim] Forged From, Other servers bouncing back

Hi spamassassin and exim lists,

Currently running on.. Exim 4.6.2-2 + spamassassin 3.1.4-1

One of the many spammers around has sent out a LOAD of email from
vcswestonwilliamsonpoh@???. Currently our server is
being over-run by the bounce backs, spamassassin is choking due to the
extensive checking it is doing and exim will not accept any more
connections.

Currently blackholing any mail being sent to this address to /dev/null
but would like a way of putting in a router before the spamassassin acl
to do this. Is this possible?

Or is there a good way I can block this in spamassassin?

Any help greatly appreciated.

Regards,
Mark