Yes — if you don't want to use the membership features in Ghost, and don't want to ask visitors to subscribe, sign up, or sign in, you can quickly disable the Portal feature from the settings in Ghost Admin.

Head to Settings Membership → Access and change the subscription access level to Nobody. This will disable Portal and prevent any visitors from being able to subscribe or sign in to your site.

If using a custom theme that includes hard-coded buttons or links for membership, these will need to be removed by editing your theme files directly.