[Palmer ofShinra]

$bbuserinfo is the key.

if ($bbuserinfo[userid]==0) {
// not logged in message goes here
} else {
// they are logged in, check to see if they've voted
}

You would use $DB_Site->query_first("SELECT * FROM eventratings WHERE userid='$bbuserinfo[userid]'");

If that returns an empty array, they haven't voted.

When they do vote, be sure to note down their userid as a check that they have voted.