There was a time when Samsung restricted the option to disable the Bixby button, but since then they changed their mind and now you can turn it off with a simple setting. Here's how to do it:
- Press the Bixby button
- In the top menu bar, tap on the cogwheel
- Uncheck Bixby Key.
After this procedure, the button should be disabled. The only way to turn it on is to go to Settings and then use the Search function to locate Bixby.