Dealing with persistant mailers

[Hendrik den Hartog]

We have a group of sites that we get 'problem' email from.

Being a school, we have students who leave after 'x'
number of years, email to these students gets bounced
with 'unknown user' messages, but some sites, e.g. some
maillists, seem to ignore these.
Also, some sites seem to reject return mail, causing
unwanted network traffic with the to-ing and fro-ing.

Some of these sites are a real nuisance!

What is the best [i.e. most economical on resources] way
to block these sites, and or any other statagies to
combat these are appreciated

Cheers!
Hendrik