While I succeeded in stopping the spambots from entering the forum itself (at the cost of my mailbox being flooded with request for activation of profiles), there was still the matter of another kind of spam which my host kindly commented on several times (each time after having shut down the forum). It was the kind of "spam" generated when the forum software automatically did send a mail to the address assocaited with a new profiled, telling them of the pending activation by Admin. Since spam-bots make use of stolen or made-up adresses, this created a vacum of pending mails going out of the forum
I'm not sure what to exactly call this, but my host called it "spam" and told me to stop it, so...
The only thing I was able to do was to switch off any automatically outgoing mails from the forum, and this was only possible if I a) marked for automatic activation of each and every new profile
or b) removed the option to sign new accounts up... 
As far as I've researched, there's really no other options and I've, so far, been unable to find any add-ons or plugins that will allow for signing up AND dismantle any outgoing mails, so as from today it's not possible to sign up any new accounts in the forum...
However, this doesn't mean that it is impossible for new people to join the boards, they just have to contact your friendly neighbourhood Admin by mail ( Hogan at dragoneers dot com), who will then make the necessary arrangements manually
In lieu that only some 5 new people come to these boards on an annuarly basis, I think its a fair trade...
I WILL be on the lookout for some way to make signing in possible and keep the forum form acquiring spam/spamming others, but for now, this is how it will go.
- Hogan