Some forums got a "Thank you" feature, where if you like someone's post, you can just click on the link or button and your name gets added to the list of people thanking for that post. I would like that more than a voting system, feels more personal, and doesn't merge disagreements with agreements.
People would still post "+1" stuff all the time because they'd want their opinions to be more visible.
But in this case, it would be fair to punish those people, as there is already a system in place to +1 people without creating a new reply.