How to block certain IP/Mac Addresses from accessing my AirPort on my 2009 iMac?
Apparently it's possible to enable MAC filtering using the Internet Sharing feature in Snow Leopard.

Here is an extended description how to do it and at the same time overcome some flaws of the sharing implementation of SL.

Essentially you have to add an additional key/array in the /etc/bootpd.plist containing the MAC as string:

    ....
    &lt;key&gt;allow&lt;/key&gt;
    &lt;array&gt;
            &lt;string&gt;0:17:43:a3:2d:45&lt;/string&gt;
    &lt;/array&gt;
    ...


Probably the opposite: deny/MAC-address works as well.

Please check the whole article and implement the respective modifications.