An upvote should actually be more about wanting others to read a substantial comment, a thank you is more personal to me. It means, hey this really touched me in a profound way (which I might even explain) but I want you to know who it affected by leaving this comment.
Which is why I don't think downvoting is appropriate for them.
So are you in favor that people should use the comments to thank everything that touches them? You can send an email to the author on his profile page, if you want to. This attitude is good and will not ruin the page for other people that are just interested in good information.
Which is why I don't think downvoting is appropriate for them.