I know that phpbb gets bombarded with spam even with captcha on. I think SMF is relatively better in keeping the spambots off the boards. Please share your experiences.
Before, I was using phpbb and yes so many spambots. Right now I'm using SMF, and I recommend that forum.
Every forum script has plugins, which you can use to prevent spam bots from spamming your forum. If you are lazy enough not to install those, not matter which forum script you get, you will eventually get spammed.
I get zero spam with myBB and SMF. I get a very low level of spam with vBulletin. I get very frequent spam with phpBB. And yes, there are mods for each of the above to help counter spam.
I've not gotten any spam yet with SMF though I want a vBulletin but funds don't allow for that at this time.
I use MyBB and don't get much spam right now. inworx is right the more popular the forum software is the going to get more spam as the spammers want to target as many boards using that scripts as possible.