If you have ever made an account at the Knot, it has probably scoured the web to look for your registries even if you never connected them to the Knot. I know there was a post about this recently but I found out how to disable it:
See if yours are public here: https://www.theknot.com/registry
If so, click "display settings" (this might be a direct link: https://www.theknot.com/registry#display-settings) and uncheck the boxes next to the registries you don't want to be listed for anyone on google to find.
I would suggest doing this instead of clicking the remove link, because the previous poster said they came back when she deleted them.
Somehow when I saw the post about it a few weeks ago I thought that I had already done away with it. Tonight I just tried to google our names registry and whelp everything was public, even though we haven't finalized it and I've essentially been using our Amazon one as my wedding shopping list :-P Luckily no one tried to buy anything yet!