have the login page store the users email address in a session variable when they log in.
If you are using security assist, this is fairly simple, edit the authenticate user server behavior on the login page, on the third step, you can chose columns to store in session variables. add the email address column.
On the page with the vote button, add a recordset to filter the email address column of the votes table on the email address session variable.
use the Dreamweaver Show region if recordset is empty behavior to only show the button if the email address is not found in the recordset.