Why not a simple caption box like most webpages do? Before you can click buy enter the text in the box... I know it's old school and this isn't some F2P Korean mmo but still you gotta admit it would be effective to limit bot activity in a lot of places..

Wanna do a shout enter the caption or be muted for x amount of time.. passing the caption removes the prompt for an hour or so..

Updating prices on a retainer.. enter the caption or you can't sell anything for a day or so.. would prevent the mass crashing of new items..

Something with crafting, gathering and maybe even combat..

Captions boxes work on most bots from what I've seen.. sure real players hate them but they are effective at controlling bots