> 
> Block the entire IP classes of those hosts for the respective ports (be aware, 
> a hostname usually point to several IPs).
> 

Yes. Do it with the acl ident or acl ident_regex

acl block url_regex "Location of messenger.txt"
acl yahoo ident hostname (or in a file)

http_access deny yahoo block 

or use the acl ident_regex  to recognize the particular pattern as hostname.

Or else

You can use the ident_lookup_access to look-up the identity of the client's ip-address 
and block them
to use the yahoo messenger!

> This is how i managed to allow yahoo messenger (by allowing that traffic).
> 
> Don't forget to check from time to time is any of the host has a new IP.

Refer this for more://
To know more about giving access denial in trasparent proxy
http://www.maynidea.com/squidguard/ident.html

Regards,
Muthukumar.

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