Typically, Google Assistant pops up when you say “Okay Google” or long-press “Home” button.
If you don’t want to use the Google Assistant anymore, then you can follow the below steps:
Open “Settings” app on your Android device.
Type “as” (without double quotes) in the search box at the top.
Tap “Assist app” (the first one) from the search results.
Tap the “Assist app” option.
That’s all. From now on, the Google Assistant won’t pop up when you say “Okay Google” or long-press the “Home” button.
Explanation: What we did in the above steps are set up the “Assist app” of the device as “None” which makes the Google Assistant stops working as your assist app.
In case, if you want to use the Google Assistant, follow the above steps and instead of choosing “None,” select “Google” and then tap “OK” in the popup. That’s all.
If you want to only stop Google Assistant from popping up while long-pressing “Home” button, then disable or change long-press action of the “Home” button. Check out this guide which may help you with that.