The first problem with 'banning' the use of curation trails (or any type of auto-voting) is that it is currently impossible for the blockchain (and thus the community) to know the difference between an auto-vote and a manual vote. However, this could be fixed through the front-end, if we require a 'captcha' style confirmation with each and every upvote -- that does diminish the user experience somewhat, though, and that would require upvotes ONLY be cast via the front-end.
A much bigger problem, though, is exemplified by this comment from @antonym:
You can't stop people from being lazy. They might as well get on the "new" page and upvote the top 5 posts twice a day. Bam! Curation done!
If a stakeholder is using one or more curation trails, that means he/she is trying to at least indirectly contribute to meaningful curation. If you take that away, you inadvertently incentivize random manual curation that muddies the curation waters far worse than curation trails.
I disagree. Stacking votes on good content would be the most optimal choice. In fact, it would be that much important. Not to mention, voting on random people means they will sel all the POB they get, which would be bad for large stakers.