Power Cinema has no delete button, instead the approach to delete files has to be different than a simple select and delete operation.
If you want to delete certain files from the software's media library, right-click a video and choose Properties and then copy the file path. After this, simply follow the path where the video files are stored and move or delete the videos from that location and use the Power Cinema's rebuild database function. You will soon notice that if the files are no longer in their original place, they will not be added to the playlist database.