Hello,
By default, In SEP 12.1 RU2, scheduled scan peformance options (in Scan Options, Advanced) are set to "Best Application Peformance". When you configure a scan with this setting, scans will start but they only run when the client computer is idle. You can change this for faster scan performance, but user applications may run slower while the scan runs.
For SEP 12.1, killing ccSvcHst.exe process would stop the Active Scan.
NOTE: You need to disable tamper protection (if enabled)
Hope that helps!!