You can deactivate most of Bixby from the Bixby Homepage, go to the upper right corner and open the menu, pick settings, and then click all those pesky boxes to the off position. Clicking the button will still bring up the homepage, but you won't accidentally trigger her constantly this way and hitting the "back" software key at the bottom will close the Bixby Homepage and take you back to whatever you were previously doing.
Once you've done all that, you can go to the Google Assistant page and activate Ok Google features. In some cases, you might have to reinstall Google if the voice activation isn't working (because of Bixby). This is what I had to do, because Bixby had messed up Google's microphone access.
I haven't used Bixby, but went to the background home page setting for the bixby page & switched it off using the slider switch at the upper right. Never even agreed to terms of service. Still comes up when pressing the Bixby button, but can use the back button to exit from it.
The real question is how can we reprogram the button to use the voice assistant of our own choosing, such as Google instead of Bixby, if we choose? Isn't Android supposed to be about configuring your phone to work the way YOU want it, not how the manufacturer wants it? If you wanted no choices, you could buy an Apple product.
You used to be able to reprogram the button early on in the S8 and S8+, but Samsung apparently didn't like that and patched it relatively quickly. I'm assuming the Note 8 shipped with that "fix" already installed.