Yes you can stop it by installing a mailfilter if you have Netscape or Widerz. Second if you can provide your own admin on the server you can reroute all mail from the sender/s to a null address, indeed with admin privilidges you can ban all traffic from named domains such as
218.0.0.0-218.255.255.255 etc.

Since you are recieving a spoofed tag AND it is USGOV you should really contact Homeland Security @your local FBI office.

If - highly unlikely - a DoD employee was doing this they would be almost shot today with security so tight... you should do something rightaway.

Hope it works out for you