Side note: ARAB actually was All Random All Bot before. It was the ARAM-variant played on the Crystal Scar between the two bot nodes.
Anyway, Riot specifically addressed the Captcha idea some months ago. Basically, it'd hurt real players more than hamper bots.
On the captcha idea, this is something that's commonly brought up, but is not an idea we're entertaining at the moment. For one, we have a captcha on account creation and we still see botting on new accounts (i.e. we know there are ways around captcha). Secondly, as a human it's really annoying to have to put in captcha codes. Bots make up a very small percentage of all players, so by implementing a captcha in order to queue up for a match (which we know bots can circumvent), we're likely just causing annoyance for human players. That's not to say we couldn't put in some logic to only present a different challenge to suspected bots using certain criteria, which is why we haven't dismissed that idea completely.