Display Condition: Hide if previously converted
If someone has already entered their information into your popup you don't want to show it again. With this display condition it will only be shown to those who haven't entered their information.
When editing your popup, first go into the Display Conditions.
Then you will want to scroll until you find Hide if previously converted. You will want to turn that on. Then you can close the display conditions (top-right corner) then save / publish your popup.