You need to change the user interface policy for Virusscan 8.7 and set a password for "Console and Miscellaneous " (User Interface Policies -> Password Options -> Other), then the user can't create any tasks.
Many thanks. Also just to clarify, there is no way to clear out existing scheduled tasks before applying the lock.
I think it is not possible to delete existing clienttasks.
You could reinstall VSE on the affected machines, that should also clear the tasks.