I know IPB has excellent spam prevention that in last 1 year I have not seen much of spam attacks. Just yesterday some smarty pant breached and posted 18 threads in one hour about fake documents etc. 

Anyways to prevent same in the future....? I mean how to limit new user to not to make more than 2-3 posts a day?

Into the group settings you can define the Posting Limits.

 

• Posting Limits

• Max items of content (posts, comments, etc.)
   per day  

  or    Unlimited

   

Thanks Tex, perfect solution.

Just wish (IPB Developers) that limit should have drop down to choose per day / per hour limit.