Varying the bot question is a good idea, which I think ought to be implemented. I also think it would be prudent for people to be asked not to give away the answer(s) to it on G2G (not intended as criticism of you, since the horse bolted on that a while ago). That probably won't make a lot of difference but it's a sensible precaution.
Also, on other, mostly forum based sites, you have to have built up a certain amount of "Kudos" - made a number of public posts before you can PM people. Maybe we could have it so you have to have created a small number of profiles, say 5 - your own and 4 others. This would be a disincentive to people who just want to spam but there is a risk they might create rubbish profiles just to be able to spam.
Alternatively, it could be having made a small number of posts on G2G - 3 or so would seem fair. Or it could be linked to G2G points by having a small point earning threshold you need to pass.