Weird, I thought there used to be a bloody great Pause button in earlier versions. But then I rarely if ever use on-demand scans (much depreciated in resident on-access AVs) other than as a result of questions in the forums.
It has either has been removed or omitted.
The Smart scan would appear to be an exception (as it would really like to sell you something), but then again it scans for all sorts of other cr4p. It has a Stop Scan button, when you do use it, there is a Keep Scanning option and then click the Next button.