Blocking potentially malicious files

We have had a recent incident where an incoming mail had an office file attachment that went to around 400 users before the spam filter blocked it. Currently SEP 12 is running in our environment.

What is the best way to block the users from opening this file, until the Symantec team has a pattern that detects it?

I would prefer to use a method that blocks based on checksum rather than filename.

This is the second such instance of undetected office file malware I have seen in a month, that has gotten thru without detection. So looking or help in preparing for further such incidents.