prevent squid being used as spam passthrough

["J Webster" <webster_jack@xxxxxxxxxxx>]

Is it possible for a proxy running on port 80 or 8080 to be used as a pass 
through or zone origination for spam email?
We have had some users sign up with email addresses such as spambot and 
other stuff recently. I suspect these are just bots signing up around the 
web but got me thinking whether a proxy could be used in a chain or tunneled 
somehow and whether that could be blocked?