I don't know any built-in function to do that, but it might be enough to set a 1 byte message size limit. That way, no message will be allowed to pass (unless its size is lesser than 1 byte, which is impossible ).
To do that run the following commands as the Zimbra user:
Mind that this is a 'dirty' workaround, so use it at your own risk
zmprov mcf zimbraMtaMaxMessageSize 1
Have a nice day (and please try to come back to the bright side)
Cine - Community Manager - ZeXtras