How do I turn off automatic product matching on my items?

From the website, click on your name at the top right side of your registry to access the account drop down menu > Settings > Registry Info. If you are logged in, a direct link to that page is here. Un-check "When possible, automatically show multiple stores that offer an item." That will remove additional retailers that were automatically added to your items. Please note: this setting can not be adjusted in the mobile apps.