I'm a grown-up. I can compose my own e-mails. Please REMOVE all suggested text!
Psst, wanna put 'Auto-Complete' out of commission?
Just put a 'z' after your cursor before you start actually writing. That pretty much kills auto-complete's "helpfulness."
Until Outlook finally gives us an actual way to turn off this annoying feature, the above workaround is your best option.