I have the same problem and have just asked the question in a new thread.
I sincerely hope that someone can figure this out. I would just like to disable the scheduled scan all together, but can't change or delete the built-in policy.
I'm not sure if what I did is correct but here is what I ended up doing. I created a new subgroup under 'My Organization'. That was the key thing that got me going in the right direction. I don't remember all the other things I had to do but, under that new group, things were editable. You just have to move any systems from the old group to that group and working out the rest wasn't too difficult. I also had to create a new deployment for new installs.
Create a new test group> move the computer the new group > go to client task catalog > policy based on demand scan > select the test group> assign the scan and change the settings as per your preferable time.
Note : at the time of installation while creating the package , you need to click on the policies under threat prevention and un check ODS - Full scan and ODS- Quick scan