I had this problem too, but now I have resolved with the captcha which is included in phpBB version 3.0.5.
For more security, go in ACP -> Generale -> Conferma visuale (for italian translation) and set:GD Codice Conferma Visuale
on Yes.Aggiungi oggetti disturbo 3D
And set Usa caratteri diversi
You can also ban an IP or IP range from the ACP by go to Users & Groups -> Ban IP's.
Italian: Anch'io sono italiano.