[Exim] Bounce mail for a particular user

Hi Guys,
I've taken over looking after a mailserver. Exim 3.30
Previously all mail to unknown users was rejected, but important mail was
rejected,
So, I asked the list and was told how to accept mail for unknown users.
Works great, Thanks to all who replied.
But, a former user, can't say worker because you would not believe how many
mailing lists he/she was subscribed to !!!
... anyway now I am getting all these subscriber emails that say click here
to unsubscribe..

So, how can I just bounce emails to that user??

Cheers,
Kerry. .