It would be great for multiple reasons

• You could UNVOTE anyone who posts you a comment without reading the post.
• You could UNVOTE anyone who makes a stupid post.
• You could UNVOTE anyone who plagiarized content.

But that will never happen because...

• Malicious people could use the UNVOTE button inappropriately.
• People would give UNVOTES for a post just because they disagree with the content.
• Many people would go into debt.
• Maybe your post is not so stupid after all!

Sorry if I cause emotion in vain to anyone!