Technically:
on *:text:*:?:{
_answer $nick # $1-
}
... How exactly will that work?
Morally:
It seems as if you'd be using this to filter *new* people entering your channel. "This discriminates against bots", no, it discriminates against users by having a BOT give them the third degree, so you can prevent BOTS joining your channel, ironic? "This discriminates against people who are dull-witted" and "thus preventing unwanted users from getting in": seems to me you think this is an advantage - I doubt you have much need for this kind of system.
Perhaps the easiest test would be to have people message you so you can invite them? You can make assumptions on peoples intelligence there too.
Many self-advertising drones infect the users mIRC and install a script to have a socket connection (and often the main connection itself) spam the nicks the main mIRC connection sees, so as you said this wouldn't stop all bots (you used the term 'human agent' as if the victim were a willing spammer, which isn't always the case).