Is there any traffic control plugin for Squid Cache? One that allows me to check if that specific user has consumed X amount of MBs or similar.
You can probably use a monitoring service to monitor your Squid Proxy. Something like Sqstat would help you to monitor the traffic per user. Alternately, you could also try Squidview which has searching and reporting functions or something called squint (I haven't used Squint so dunno much about it).
Hope this helps...