After i installed BBAntiSpam, the amount of spammer in Indonesian Linux Forum has decreased a lot. This is a proof that BBAntiSpam is very effective for fighting spammers (at least for now).
The only thing you should consider is updating the questions regularly. Since it's based on random questions, there's a chance that spammer who had modified it's spam scripts could answer the questions provided by BBAntiSpam. Adding questions in BBAntiSpam is very easy, since it's array-based, so you can see the pattern on the configuration file.
I will see for the next few weeks and if there's no leakage, i'm considering to change the registration method again to email, so that users who has just registered can activate their account and directly use the forum without waiting for my approval.