Which theme are you using?
The free Shopify made themes don’t have any pop ups built-in, so if you are using one of these, it’ll be an app that is responsible for the pop up. Let me know if this is the case, and I’m happy to help you track down which app it is, and how to get it fully uninstalled.
However, if you are using a third party theme, please reach out to your theme developer directly, and/or check their help center to determine how to disable the pop up function. They’ll be your best resource to get this resolved as quickly as possible.
Let me know if you have any other questions!