- Enable developer options on your phone and turn on USB debugging.
- Connect the phone to a computer with a USB cable.
- Do this (on OS X):
brew install android-platform-tools
adb shell
pm hide com.sec.android.inputmethod
- Restart the phone.
After doing this, I was able to turn off developer options (and restart the phone again) and the Samsung keyboard was still not listed under available input devices.
The Samsung keyboard is still installed, but now I only see it when booting (to enter my password to decrypt the phone). Once booted, it doesn't show up under input devices anymore.