I have faced this issue often.
I schedule a scan and next day when I check, I find that the status is "running". I try to kill the process "browser.exe" but still I am unable to change the status of scan.
The WebInspect UI offers the ability to Stop the running Scheduled Scan. You would find these controls on the Start Page tab of the primary UI >> "Manage Schedule" button in the upper center of that screen.
From the Help Guide:
Stop a Scheduled Scan
To stop a scheduled scan, select a scan that is running and click Stop (or right-click a running scan and select Stop Scan from the shortcut menu).
If you wished to kill it directly, the specific process spawned by WebInspect is named WI.EXE, same as if you ran the scan from the Command Line (also in the Help Guide). The various browser.exe processes you see go along with the scan, but it is just spawning them to parse scripts. All of those browser.exe should be killed, but they should end once the scan itself ends.