Unfortunately, built-in phpBB's "Write to administrator" form lacks any sort of protection or captcha and spam bots are annoyed me to write their letters about their marketing or even advertize some weird porn crap. Therefore, I'll disable this completely.
Instead, I'll make on a quick hand the alternative "Write to us" that will be protected from off spam bots.
Added after 44 minutes 10 seconds:
Okay, just now I have made new feed back form!
I hope, it will work fine!