The spam is not automated, they are individuals who actively seek forums such as this in various ways. So unfortunately captcha would not deter this method of spamming.

What can be done is a user approve. The user would need to be reviewed by a moderator prior to having the capability of posting. However, this would likely not be appealing to a real end user. Also, there is relatively little that can be determined before the user is approved, so we may not know their intentions until after they are approved, furthering the cycle of cleanup.

Some of the bigger forums do have 24/7 monitoring. If some of the more experienced members such as yourself may be interested in moderating email dave - info at rugrag dot com