I notice spammers are posting on this forum yet it has captcha installed. Any idea how they are able to post? Is it something that I will need to be concerned about on my own forums?

Captcha is only a weapon against automated spam. Manual spam could pass any guard and any filter you may have installed. That's why, from time to time when I'm completely offline and doesn't follow it, this forum also may have to bypass some manual spam messages.

There is no absolute solution against spam except pre-moderation, but if the forum is opened for everyone incl. guests, there always will be spam messages, this is logical.

Here is bit of other discussion on this topic.