Using Pop Up or Slide in as a reveal tab

Can the close option from either the Slide In or Pop Up be removed to force a visitor to sign up/register? I want it to work like a reveal tab, where they only have to sign up once, but have to sign up before they can see the site content. I haven't decided if I want to use it this way just yet, I'm asking ahead of time to see if this is easily done, or if it's even an option. Thanks!