Originally by: clavo oxidado
Possible way would be to use a system similar to the anti-spam techniques that most posting sites use. eg type the characters displayed in an image. Users that dont respond should be automatically logged off.
You are suggesting chatcha's, this have been suggested before, several years ago you could buy bot (for setting up web mail accounts) that had a 60% success rate on first attempt.
Also the large isk farmers don't leave their bots unattended they have herders who would also be able to solve the check you propose.
Do a search for the full details.
However I do like the basic intent