This already does happen :
And you could penalize people on their curation rewards if they are abusing the voting system.
After 20 votes in a 24 hour period there's a steeply curved drop off of voting weight. If you want to see your current voting power just look yourself up on steemstats.com created by @jesta. It allows you to track any users voting weight and shows how many votes they've thrown out in a 24 hour period. I use it daily because it has all kinds of embedded tools that scrape the blockchain for all the info you could possibly need. I honestly use it more than the built in user dashboard because it has more functionality right now and a much better user interface.