Hi.  I would like precise instructions for effectively blocking AOL
AIM and iChat on Mac OS X tiger.

it would be hard to block as if you tried to block the port, the user
could just change the port that iChat or AIM operates on, and you
could change the user permissions for that particular program, but it
wouldnt stop them from downloading it again

Just delete it.(aim, aol, ichat)

If iChat is installed, drag it into the Trash Can.
If AIM is installed, drag it into the Trash Can.
Ditto for any other instant messaging clients.

It is important to know the nature of your needs. If, for example, you
are trying to limit your child's access from iChat and AIM, log in to
an account with administrative permissions, open system preferences,
and click on the accounts pane. Click on the account you are trying to
disallow, click on the security button, and make sure "Allow user to
administer this computer" is unchecked. This will allow you to
administer the user's abilities, and keep the user from changing
prefrences and re-installing these apps. Under the accounts name it
should say somthing to the effect of "Managed" and not
"Administrator". Then click on the limitations button, and click on
"This user can only use these applications:" Und use the pane to
disallow iChat and AIM.

This should solve your problem, unless you haven't created multiple
accounts. I highly suggest that you talk to the user you are blocking
along with any actions you take, because almost any limitations you
make could potentially be sidesteped.