I see, sorry about that I was writing a paper for the last 10 hours until... I've banned this bot and removed all threads. I guess you can give infraction to users, and once infraction points reach a limit a user will be banned. So next time when this happens(assuming I am not online), send infraction repeatedly(usually 3-5 times is enough) till the bot is banned.
And thanks for letting me know, this bot was still visiting the forum even after I banned it. The good news is that it wont be able to do any damage now.