If you are using webmail, go into Settings and under the Spam Controls section under the Spam Filters by Address you can specify email addresses to block using the Custom option.

If you are using AOL software, under the Mail drop down menu click on Block Unwanted Mail.  In the lower half of the Mail and Spam Controls box, you will see a link called Sender Filter.  If you click on Edit next to that link you will be prompted with a box that will allow you to specify email addresses to block.