$sql = 'SELECT *
FROM contact
WHERE id NOT IN (SELECT mycontact.id
FROM contact AS mycontact
WHERE 1/* your condition*/)';
$rows = R::getAll( $sql );
$contactsMinusMyContacts = R::convertToBeans( 'contact', $rows );
This code should be the best performing, as it runs nearly entirely on the database, which is the best place to work with sets of data. It first selects all contacts, then subtracts "my contacts" and returns the result. RedBean then converts it back to beans.