Another classmate and I (and there may be others) have received ads from someone using another classmate's name and email address. Ads for Canadian pharmacies or some other issue. What can be done to assure that other classmates will not receive such emails from someone posing as another classmate?

I think that the other classmate's system has become infected with a virus that is "stealing" his contacts and then sending out spam. It's a fairly common problem.

To assure that other classmates will not receive emails from his system is for them to remove the malware.

IOW, it they need to run some anti-malware/virus program to remove the program, otherwise it will continue to misbehave.

If, as I think, the other classmate is not on his contact list the only source of the email address of the sending classmate is our website. What then?

Need more information. If you look at the underlying routing data in the email you received it might answer the question. How to see this varies by how you receive your email. Google for how to do that for your email and then post what you find.

It looks something like this for me (gmail) for your response here - note that it says where it came from

Delivered-To: me@mydomain-name.com
Received: by 10.204.239.202 with SMTP id kx10csp136083bkb;
Wed, 29 Jan 2014 13:48:16 -0800 (PST)
X-Received: by 10.182.135.194 with SMTP id pu2mr8360093obb.38.1391032095558;
Wed, 29 Jan 2014 13:48:15 -0800 (PST)
Return-Path:
Received: from mail.classcreator.net (mail.classcreator.net. [69.41.172.242])
by mx.google.com with ESMTP id qd1si1853663oeb.122.2014.01.29.13.48.14
for ;
Wed, 29 Jan 2014 13:48:15 -0800 (PST)
Received-SPF: pass (google.com: domain of noreply@classcreator.net designates 69.41.172.242 as permitted sender) client-ip=69.41.172.242;
Authentication-Results: mx.google.com;
spf=pass (google.com: domain of noreply@classcreator.net designates 69.41.172.242 as permitted sender) smtp.mail=noreply@classcreator.net
Received: from www.facebookpost.com ([69.41.171.62])
   by mail.classcreator.net (mail.classcreator.net)
   (MDaemon PRO v12.5.
   with ESMTP id md50058159886.msg
   for ; Wed, 29 Jan 2014 16:48:11 -0500
X-Spam-Processed: mail.classcreator.net, Wed, 29 Jan 2014 16:48:11 -0500
   (not processed: spam filter heuristic analysis disabled)
X-MDRemoteIP: 69.41.171.62
X-Return-Path: noreply@classcreator.net
X-Envelope-From: noreply@classcreator.net
X-MDaemon-Deliver-To: me@mydomain-name.com
Date: Wed, 29 Jan 2014 16:48:05 -0500 (EST)
From: ClassCreator Admin Forums
To: me@mydomain-name.com
Message-ID: <1730030012.53633.1391032085691.JavaMail.cf2@69.41.172.242>
Subject: New Response: Hacking
MIME-Version: 1.0
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 7bit
X-Mailer: ColdFusion 10 Application Server

Tom Pearson wrote:

Thank you for the information , I will have the classmate check it out.

I receive this type emails from classmates who use a "YAHOO" based email. One of the reason for the 'hacks' is their passwords are not complex. My advice would be to use something like "Firstchildname1Secondchildname2Thirdchildname3" or grandchild or a complex sentence "IliketoplayGolf". These passwords are not easily hacked.

Thanks, I'll check it out.