Hot to ban "::whitespace::{4,}.(com|exe)" in archives?

[Ralf Hildebrandt]

Currently my users are receiving trojans camouflaged as legal threats,
a zip containing a "legal_document.doc                 .exe"

How can I block those?
-- 
Ralf Hildebrandt                   Charite Universitätsmedizin Berlin
ralf.hildebrandt at charite.de        Campus Benjamin Franklin
http://www.charite.de              Hindenburgdamm 30, 12203 Berlin
Geschäftsbereich IT, Abt. Netzwerk fon: +49-30-450.570.155