There isn't a captcha system in UBB.Classic v5; you'd really need to upgrade your system to help thwart spammers.
EMail validation and a captcha are both in latest versions of the UBB (that'd be UBB.Threads7).
The reason Infopop/Groupee directed you here is because Rick (the UBB.Threads lead developer and UBB.Threads original developer) now owns the UBB.