How can I block Instant Messengers?

It's practically impossible to prevent, since many will use port 80. You can block ports for some, and IP addresses for others, but the IM industry is always making evasive changes to your efforts to block.
However there are devices that will stop IM
check out www.akonix.com they have an appliance to filter out IM

If using I.E. as browser, why not put the path to the login site under LOCAL INTRANET>SITES and then add the path to block. If you have I.E.A.K. for I.E. then you can set option in the rights to remove the TOOLS option in the browser so no one cango in and change permissions to sites.

First would be to take control of the PC's using Policies or Permissions. You don't mention what OS you are running, Win 95/98/NT/2000?

All of these OS's have ways to limit what a user can do. First I would remove the messaging software from the PC's and then limit the users ability to install or run software. This is the first line of defense and best if you simply do not want people using these services.

If you cannot do this, then you should look to the firewall to block well known ports for the various Applications check out (http://www.practicallynetworked.com/sharing/app_port_list.htm) for a good primer on the commonly used ports for most of these programs.

Second is to lock down specific IP's or domains from access.

The other suggestions above, are good to control what IE can get to and what it can't. If you implement this and then limit what your users can do you should be able to very effectively shut this traffic off.

Extremely simple but very effective way to deny access to web sites is to use the built-in HOSTS and NETWORKS files. Edit these and for every site you want to block, such as in msn.com, add a line to the HOSTS file, thus;

127.0.0.1 msn.com
127.0.0.1 aol.meesenger.com

This will fool the IP stack into thinking that the web site is actually hosted on the local PC and as such will bring up a 'Page Not Found' (the much loved Page 404). Use the NETWORKS file to explicitly allow certain networks, thus;

LONDON 192.168.254.2
CAMPUS 192.170.200.1

These 2 are quite useful in restricting site abuse and misuse. Remember, however, the files have no extensions and must remain so. As it is, my HOSTS file is 25Kb big and stops most ad-servers and pop-ups...

This question was closed by the author