There should be a settable time frame of membership this can be applied to.
Regular, long term members will often send a number of messages in a 24 hour period, but scammers are typically new members.
Ok, I own a number of big boards with many registrations every month. (probably 1000 actual registrations a month between the different forums not including the ones that are blocked by my methods)
Here is what I do to combat spam. These methods block most spammers from registering and help you...
Many scammer are in time zones that make them obvious, such as West Central Africa --- I would like to be able to search by time zone to help identify these users