This isn't gonna be a very long one.
High alchemy is the current title holder of 'most autoed' with at least few bans for it every week. While the bots are quite easily caught and reset, I feel it really does need to be discouraged more. Simply reducing the exp you get from it would probably drive more people to actually use combat spells to train magic, as it's meant to be. Additionally, some items that are commonly alched could also be made non-alchable, these would be items such as runes, feathers and fishing bait.
Alch values could be slightly turned up to compensate for exp loss.
I know it's gonna destroy anyone trying to make a 99 mage skiller, but they could always use superheat to train. I think people autoing alching is more of a problem than the discomfort of 99 mage skillers.
I'd be really interested to see why people would disagree to this, please post.