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 the Settings app on your Android device.
Type as in the search box at the top.
Tap the 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 is 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 pop-up. That’s all.
If you want to only stop Google Assistant from popping up while long-pressing the Home button, then disable or change the long-press action of the Home button. Check out this guide, which may help you with that.