AIM 6.1 allows You to set up AIM to show a curtain message for people not on your Buddy List. This allows you to choose whether or not to accept messages from people whom you may not know. If you accept the message, and do not wish to receive further messages from a particular screen name, you can block this screen name right from the IM form.
Alternatively, you can report the IM as spam. If you do read this message, you may still report the sender as spam and choose whether/not to block the user from sending any further messages to you.
Report Spam reports this screen name to AIM so that we may determine if this user is a spammer. You will not receive another IM from this screen name until you exit AIM and sign back on.
Blocking the user will add the screen name to your privacy block list and the screen name should not be able to send IMs to you in the future.

Accept an IM at the Curtain Message
When you get an IM from someone not on your Buddy List, you will get an IM window with 3 choices:

Show the IM: this will open the IM window to see the message
Close the IM: this will close the IM window without displaying the message
Report IM Spam: this will report the screen name to the AIM spam system

Block or Unblock Screen Name to Your Buddy List
Launch the AIM software, then sign on using your screen name and password.

The block or unblock screen names to your Buddy List:
1. Tap the F7 key or click the Edit menu to open it, then click Settings....
2. Click the Privacy tab.
3. Choose one of the following options by clicking it.

Allow all users to contact me: When you select this preference, any AOL or AIM user can see that you are online and can send you instant messages
Allow only users on my Buddy List: When you select this preference, only people whose screen names you have added to your Buddy List window can see that you are online and send you instant messages.
Allow only the users below: When you select this preference and add names to the list, only the people whose screen names you add to the list can see that you are online and send you instant messages.
Block all users: When you select this preference, no one can see that you are online or send you instant messages.