1.1 as best I can tell. And the only thing to do with spam check is a checkbox with the description "Disable spam check on posts". It's under a general category, and nothing I can find in membergroups, permissions, or board configurations allows me to change that.
Are you able to install mods for the forums, or does smfforfree not allow that? Otherwise, people have been describing guest-only captchas with
http://custom.simplemachines.org/mods/index.php?mod=907A cursory glance didn't suggest we can, but I'll continue looking.
EDIT: Took a look at the support forum, and it looks like smfforfree can't handle mods:
http://www.smfsupport.com/support/suggestions/(suggestion)modifications-kinda-like-simplemachines/