I still like the idea of players being able to "tag" or "flag" suspected bots.
Since Jagex already has a right-click report function why not incorporate a "bot-tag". Say if a player gets tagged, 5 or 10 times in a certain time period, then they should have an interactive event triggered in which they would have to do something to prove they are players.
This wouldn't be easily abused if there wasn't any reward except ridding your training grounds of pesky bots. Also this could be an interesting way to incorporate an "ethereal" form for players who ignore the event.
For players who are repeatedly flagged as bots who aren't botting, there could be an increase in "tags" needed to trigger event for those players, for completing the interactive event multiple times.
To rid of bots Jagex could then monitor ethereal accounts who have not logged out or finished said event.
I think with a little work this could be a way to get the players into the fight against bots. I for one am anxious to do more then just kill bots at green dragons, and steal bots kills at Mature Grotworms. Let the players help too! Jagex isn't alone in this fight.