It is possible that when making a purchase or entering an email address on your site, a customer is automatically signed up for a Glow account.
How It Works
Glow leverages the account creation and management feature by Shopify. This means that if someone creates an account on your Shopify store, that account is automatically picked up by Glow. This is good news, because it means you are growing the biggest data asset your company has--it's loyalty program, without forcing your customer to create multiple different accounts.
Shopify also allows your store to enable a feature that automatically forces people who purchase a product to create an account at the point of purchase if they don't already have an account.
Additionally, some of the other 3rd party apps that you can install on your Shopify store also have this same feature.
How To Turn It Off
Go into your Shopify store settings --> Checkout and make sure you select the option "Accounts are optional".
Go into your Shopify store settings and/or app settings and make sure all options that contain language like "automatically create customer account" are marked as "no".