I left MyBB due to the spamming issue, captchas including Recaptcha can't stop spam and I got like thousands everyday. I'm little nervous when started using the Q2A but I rarely get any spam posts but I still get them. I'm not sure its from bot or real humans though.
Here are some of the options which are used by other forums to control spam?
1. Disallow posting content which has link in it.
Used by: Fluxbb, Simple Machines Forum and few more. Those forums has different options to configure but working is basically the same.
2. Avoiding Bots from Creating An Account.
Used by: phpBB and few more.
3. Hiding Links From Search Engines & Visitors (non-logged in users).
Used by: MyBB and few more. Even though spam links are nofollow, they are good to be hidden.