How do I hide the login button from the Customer portal?

If you do not wish your customers to login to your portal, but only to view the content which is made available on it, you could hide the login button from your portal. 

To hide the login button, you can use the following code under the Stylesheet section:

a[href*='login']

{display:none;}

This can be found under Admin --> Channels --> Portals --> Customize --> Edit theme > Stylesheet and would be available from the Garden plan onwards.