for admin to block im sites with hosts file,
1.visit http://www.mvps.org/winhelp2002/hosts.htm to dl the hosts.zip
2.paste the following url to the hosts file with notepad
3.run mvps.bat (the files will finally be backup and replace at something like C:\WINDOWS\system32\drivers\etc
4.restart browser

here are some of the popular im sites

127.0.0.1 localhost
#127.0.0.1 messenger.hotmail.com
127.0.0.1 dp.msnmessenger.akadns.net
127.0.0.1 webmessenger.msn.com
127.0.0.1 get.live.com
127.0.0.1 www.facebook.com
127.0.0.1 www.icq.com
127.0.0.1 www.wablet.com
127.0.0.1 www.wablet.us
127.0.0.1 www.radiusim.com
127.0.0.1 www.messengeradictos.com
127.0.0.1 express.instan-t.com
127.0.0.1 www.oyco.com
127.0.0.1 www.imunitive.com
127.0.0.1 www.koolim.com
127.0.0.1 www.messengerfx.com
127.0.0.1 www.communicatetube.net
127.0.0.1 www.mabber.com
127.0.0.1 www.mabber.us
127.0.0.1 msn2go.com
127.0.0.1 msn.audiowatcher.com
127.0.0.1 www.imhaha.com
127.0.0.1 www.e-messenger.net
127.0.0.1 www.ebuddy.com
127.0.0.1 www.iloveim.com
127.0.0.1 www.msnger.com
127.0.0.1 messenger.msn.com
127.0.0.1 messenger.yahoo.com
127.0.0.1 webmessenger.msn.com
127.0.0.1 mob.e-messenger.net
127.0.0.1 harare.e-messenger.net
127.0.0.1 wap.e-messenger.net
127.0.0.1 boston.e-messenger.net
127.0.0.1 hongkong.e-messenger.net
127.0.0.1 houston.e-messenger.net
127.0.0.1 bangkok.e-messenger.net
127.0.0.1 hongkong.e-messenger.net
127.0.0.1 e-messenger.net
127.0.0.1 start.e-messenger.net
127.0.0.1 macau.e-messenger.net
127.0.0.1 tokyo.e-messenger.net
127.0.0.1 chicago.e-messenger.net
127.0.0.1 washington.e-messenger.net
127.0.0.1 orlando.e-messenger.net
127.0.0.1 homer.e-messenger.net
127.0.0.1 mail.yahoo.com
127.0.0.1 www.msn2go.com
127.0.0.1 www.phonefox.com
127.0.0.1 www.msn2go.com
127.0.0.1 www.onlinemessenger.nl
127.0.0.1 www.Jaimy.us
127.0.0.1 www.universia.es
127.0.0.1 www.wbmsn.net
127.0.0.1 kickme.to
127.0.0.1 www.mastaline.nl
127.0.0.1 www.universia.e
127.0.0.1 chat.ohearn.ca
127.0.0.1 www.e-messenger.cl
127.0.0.1 www.msnanywhere.com
127.0.0.1 iloveim.com