AIM have application for mac os already which works really cool
Go here download it and install it.
http://www.aim.com/get_aim/mac/latest_macosx.adp
Then you can set the (privacy)settings

Click on Aim Button >> Choose Preferences
Go to privacy tab

Choose the options you like the most.I have selected the option “Allow only users on my Buddy List”. Thats the best for me.

