Try modifying the default messages view where the preview pane is off.
View > Current View > Define Views > select Messages > Modify button > Other
Settings button > set reading pane off
Note: I would start Outlook 1 time with the /cleanviews command line switch.
This will delete all custom views and reset the built-in ones back to their
defaults. Then use the above to turn off the reading pane. That should
make it global for you.