Deletes are not allowed unless they contain a "where" or "like" clause.

I am sorry , I don't why I did not have look to your code, before answering.

I never did delete with using QB, so I am not sure your code produces the right SQL code. 
try running

Code:

echo $this->db->get_compiled_delete('eventi_partecipanti_accompagnatori');

and check the SQL code it generates

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I am sorry , I don't why I did not have look to your code, before answering.

I never did delete with using QB, so I am not sure your code produces the right SQL code. 
try running

Code:

echo $this->db->get_compiled_delete('eventi_partecipanti_accompagnatori');

and check the SQL code it generates

Also I see you can remove 'left' arguments from you joins, as where clause will eventually filter those NULL fields.

I insert the query without QB and the query it'ok run on adminer ( similar to phpmyadmin)

Code:

$sql = "DELETE eventi_partecipanti_accompagnatori
                from eventi_partecipanti_accompagnatori
                join accompagnatori_club on accompagnatori_club.id=eventi_partecipanti_accompagnatori.id_accompagnatori_club
                JOIN user on accompagnatori_club.id_user=user.id
                WHERE user.id= $id_user
                AND eventi_partecipanti_accompagnatori.id_eventi = $id_eventi";
        $res=$this->db->query($sql);
       
        if($res){

            return TRUE;

        }else{

            return FALSE;

        }

but same problems...