You entered the amount you were willing to pay when you placed your bid.
The auto bid will stop when and if another bidder exceeds your hidden proxy.
If you're not outbid, congratulations, you got the item at the price you were willing to pay or for a little less if you're lucky.
I assume you mean how do you stop other people from autobidding, why would anyone want to do it to themselves?
If you are not interested in winning an auction but like to bid, just bid the minimum amount allowed and hope someone outbids you, otherwise the entire purpose of automatic bidding is to help you win.
You can't stop others from taking advantage of what is perhaps eBay's best bidding feature.