I don't suppose it's absolutely possible to stop them voting on their own stories.
I have the one account, only ever had the one account. I didn't used to vote on my own stories, until I tried it once to see what happened, and shortly after that a readers told me, "If you don't think your stories are worth voting for, why should anyone else?" I had no answer to that, so I have since then, because it's only ever one vote.
My son lives with me and we share the Internet. I know he opened an account at SoL and FS, but I don't think he's used it in a long while, so not sure about it's status. He spends more time with a couple of FanFic sites than at SoL or FS, always has. I don't think he's ever voted on a story, but don't know for sure. I keep passing on story titles for him tor ead, and he keeps saying "sure, alter," but never talks about them.