Pop-ups on a page can be removed by selecting one of the two pop-up removal options under Advanced Options.
Omit pop-ups from the screenshot
The first option, “Omit pop-ups from the screenshot” will attempt to remove the pop-up from the screenshot. This works in most cases and should be tried first.
Omit specific elements from the screenshot
The second option, “Omit specific elements from the screenshot” should be used when the first option fails. This option will allow you to enter CSS selectors for the elements you’d like removed from the screenshot. For example, you can enter “#pop-up, #pop-up-modal” and this will remove the elements with matching selectors.
Information about CSS Selectors: https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Getting_started/Selectors
Note: You will need to click Refresh snapshot image if this setting is applied to an existing snapshot.