Transparent Proxy & limit Instant messengers in schedule

[chchang]

Dear All
I want to limit some users connect to MSN or GoogleTalk in work time (0800 - 1200 , 1400-1800)
But in their own break time , they can connect MSN or GT without any limit .

First , I add an alias to contain all MSN Login site I know (207.46.x.x …..etc )
in my test , the computers been block traffic to MSN will not be able to ping , to telnet to the MSN Sites (which is exactlly I expect .) ,
and  I  setup a transparent proxy with squid (but no SquidGuard) ,If I add some policy to block all traffic to MSN Login Site , because of the transparent Proxy , users can still connect successfully(not only MSN Messenger , But also www.msn.com.tw which should not be connected).
and , Yes , I try to add some limit in squid.conf directly , this will limit the IP address had been banned not to connect to IM "all day" , But I Just want to block the traffic in specific period.
so , Does anyone have any idea how to block IM with transparent proxy in specific period ?

thanks for response and forgive my poor English :<