The best fool-proof way is to create a new folder in Outlook and dump the emails you're wanting to keep into that folder.
The whole idea of "syncing" is you're synchronizing the Outlook inbox. So if you don't want to sync it then other than outright not syncing the phone and outlook, you can do the above. You could try to set up your outlook to push sync items to your phone but then when you delete your phone emails, your outlook will push those deleted items upon re-synching.