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.