As long as the post payout is positive, then the user's reputation score will not go down. If someone downvotes a high paying post (but not enough for the payout to reach zero) then it just means the person's reputations score will go up less. (Since the post has a positive payout, the overall affect on the reputation from all the votes combined is still for it to increase.) Does that make sense?

Also, if you have a lower reputation than someone, no matter what you do - you cannot make their reputation go down. Only someone with a higher reputation can cause a score to go down.

There are a lot more details here (search for "reputation"):
https://steemit.com/steemit/@shenanigator/thanks-to-your-help-the-official-steemit-faq-is-now-complete

If you have any other questions, let me know :)