User Configuration > Administrative Templates > Microsoft Office 2016 > IME (Japanese)
User
HKCU\software\policies\microsoft\ime\shared\16.0\sharepointdict
This policy setting allows you to turn off updating for SharePoint dictionary of Microsoft IME (Input Method Editor). This policy setting applies to Japanese Microsoft IME only. If you enable this policy setting, SharePoint dictionary is not updated, and you cannot add a new SharePoint dictionary. A SharePoint dictionary that was added before enabling this policy setting is used for conversion. If you disable or do not configure this policy setting, SharePoint dictionary can be updated and added.