As reported e.g. in
this thread, there's a problem with scheduled scans which appeared just recently. The scans start automatically even when they shouldn't (at the particular moment), and they cannot be stopped - once the task is manually stopped, it starts immediately again.
I believe that the problem occurs only for
monthly scans, scheduled for the
first day of the month between 0:00 and 1:00.
In that case, the scheduled task is started immediately (and as soon as the scheduling window is closed), and when the task has finished or has been stopped, another instance is started right away.
The problem started yesterday (29.3.), it's also happening today and it will also happen tomorrow.
After that (i.e. since April 1st), it should stop.
As a temporary workaround, I suggest either to disable the scheduling of the particular task, or to change the scheduled time outside the 0:00-1:00 range.
To stop a running task (and prevent it from being restarted), it should be enough to [temporarily] change the system date a few days ahead (to an April date), or a few days back (before March 28th) - then it should be possible to stop the task normally.