Would it help to require a statement of why applicants want to join which could not be filled out by a machine properly, then you could glance at this box to see if it was a legitimate subscriber for any that make it through and quickly delete those that are not legitimate.
So far we haven't had any spam signups to our recently moved group. Back in the early days we took the decision to ask applicants to reply giving a name/nickname, age and location before membership was approved. We don't have a large membership, nor do we expect more than a couple of signups a month (if we're lucky!) but spam signups won't respond to a request for details. We had a long period with this issue at yahoogroups (when we were busier with signups) and it was easy to weed out the spammers.
