Wirelessly posted (Blackberry8700c: BlackBerry9630/4.7.1.40 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/105)

The average spammer can break CAPTCHAs pretty easy these days (google xrumer for details). Unfortunately, as the site owner, the best solution is probably to require email verification before posting plus a check on IP address of registration. It can get to be a pretty painful process with this manual intervention.