codice:
$query = "SELECT *
          FROM ".$tabella."
          WHERE (email LIKE \"%$word%\"
          OR nominativo LIKE \"%$word%\"
          OR id LIKE \"%$word%\"
          OR nickname LIKE \"%$word%\")
          AND validato = 'y'";
Essendoci una AND le condizioni in OR devi metterle tra parentesi. Altrimenti diventa:
codice:
WHERE 
email LIKE \"%$word%\"
OR nominativo LIKE \"%$word%\"
OR id LIKE \"%$word%\"
OR 
(nickname LIKE \"%$word%\" AND validato = 'y')";