To be fairly honest with u all the 3 have similar level of security where in my personal opinion I believe mybb is the easiest to deal with for newbies.
Its very friendly to setup, manage groups and permissions not to mention you can easily make a blog embed on it together with membership stuff.
It have plenty of plugin and its hell easy to make plugins for it as well.
phpbb if I am not mistaken is on the market for longer time however I am not much of a fan its hard to manage on the long run but then again that's my personal opinion.
I found SMF pretty crappy to be fairly honest but some people like it.
If I had a order of like it would be IPB > vBulletin > MyBB > PHPBB > SMF those are the most used ones.
EDIT: As for spam, there is a great plugin for MyBB, that people will post but only after 1 of their posts is approved the rest will show up, so if the forum is going to be managed by you and other mods that should be enough to retain spam.