One method might be to setup a postmaster filter with Header 1 set to From and the value equal to the email address and then configure the Set Header 1 to X-OTRS-Queue and the value equal to a trash queue.

Then, create a generic agent to automatically delete items in the trash queue every 10mn

Nathan Campbell
Dallas Symphony Orchestra

From: otrs-bounces@otrs.org [mailto:otrs-bounces@otrs.org] On Behalf Of Jean BROW
Sent: Wednesday, August 18, 2010 3:13 PM
To: otrs@otrs.org
Subject: [otrs] Spam from an user

We having an @hotmail.com user who sending spam to our OTRS system. How can we block his email address (we cannot block hotmail.ocm domain on the server).

Thank you.