The initial situation is: a user clicks on a preview image that triggers an overlay to start a video. The overlay has an "X" button in the top-right corner that stops the video and closes the overlay.
A client's idea is to disable the close button for 30 seconds to force the user to watch the ad that precedes the video. After the waiting period the regular close button would appear.
While this idea is questionable itself as far as UX is concerned, the question is related to the close button: Should the close button be disabled, i.e. greyed out and made non-clickable or should it be removed? Or to put it differently: which option is less confusing for the user, keeping in mind that bounce rates, pages per visit and the percentage of returning users are important KPIs?