Android - Is there a way to completely disable all Bixby functionality without rooting the S9+?

This article has a useful guide to disabling bixby.

Disable Bixby Voice

Go to Bixby home. Click the three vertical dots then settings. Uncheck Bixby Voice

Disable Bixby Button

Go to Bixby home. Click the cog. Uncheck Bixby Key

Remove Bixby from Home screen

From home screen, hold down on empty space. Swipe right until you see Bixby home screen. Uncheck Bixby Home.

This other article covers remapping Bixby button. You will have to keep the button enabled. Skip step two in the previous article.

This one is longer, but basically you download an app that will remap the key to what ever action that can be done.

EDIT

As of Android Pie (9.0), this guide is no longer accurate to disable Bixby. The Bixby button can still be remapped using bxActions(free) or Tasker beta(paid)(thanks beeshyams) and Bixby Home can be removed from the home screen. However, disabling Bixby completely will most likely require removing via ADB or possibly using Package Disabler Pro+(Paid app).

Yes this can done. I recently updated guide to remove Bixby using adb.

Assuming you've adb installed on your machine, you can run following command to get packages related to Bixby.

adb shell pm list packages | findstr "bixby"
or
adb shell pm list packages | grep "bixby"

Next step is to uninstall those packages for the current user (that is you). An example for one of the packages:

adb shell pm uninstall -k --user 0 com.samsung.android.bixby.agent

Do this for all packages and you will have everything related to Bixby removed from your device. I just verified with my S10 Plus device and it works fine.

One caveat is: The application is being uninstalled for the current user (--user 0) – not system wide. Root access it needed to do system wide un-installation. This could also mean that whenever you’ll upgrade or factory reset your device, the bloatware would kick in again. Also button remapping might also not be possible.

If this sounds very technical to you, you may want to read the guide for step by step instructions.