Well MS might have done that deliberately since they would want the end-user to use their service atleast once a day. If you do not use your MS Account for email then just uncheck the Email option from email and accounts under settings
Unfortunately my wife has requested that the email automatic sync on her phone is turned off by because of stress. The phone is however the only way she can check the email but it needs to be done manually so that she can manage the stress levels.
Why not just turn off all notifications for the email account. Let it sync automatically but check it whenever she wants to. It's probably the notifications that stress her out.
You also could set "download new content" on daily.
If so she can open the e-mail but she will not see new messages. Only if she push the sync button on the bottom she will see the new messages.
It's the closes you can get to manual sync in your case.