Why are there customers in my list that I never added and who never signed up?

These come from guest checkout. When someone places an order on your storefront without signing in, Wcart looks for a customer with that email:

• If none exists, it creates an account for them, generates a password and emails them their login details, so they can sign in later and track the order.
• If an account already exists, the guest order is not linked to it automatically.

So every guest buyer appears in Customers → Manage Customers. You can manage them like any other customer: block, edit or delete from the list or their profile.