It's now possible to turn this off in the Release Preview. At least the top/left last window preview (which seems to be the most troublesome).
Not quite sure the best way to navigate to it, but you need to go to
PC Settings >
App switching and turn off the first option that reads
Allow switching between recent apps. As silly as the option is worded, don't worry, it doesn't disable ALT+TAB or anything.
I got there from the home (metro) screen:
- Click your name on the top right
- Click change account picture
- Click General on the left
And you're there...
ETA: This has a side effect that you may or may not want. I realized after turning it off that I could no longer ALT+TAB to Metro apps. That might be a good thing for some people, but it sure would be nice if there were two separate options (1 to turn off the hot corner, and the other to turn off alt+tabbing).