There are two ways to do it.
One is simply removing all letters from both “static_keys” and “random_keys” on the front of a card (you will need to do it for each card type individually since it is per-card setting):
For more details on setting up the keyboard please refer to this post.
The first method leaves the “Hint button” on the screen. If you want to remove it as well, you can add “off” class to it:
You can actually add this class to any element on a card to disable it (I wrote more about adding classes here), which is a better way to get rid of elements than deleting parts of code because it is non-destructive and can be easily reverted. So for the second method, you can add this class to the keyboard itself, without any other changes on a card:
(just make sure to do it on typing cards only, disabling the keyboard on multiple-choice questions will remove the answer buttons, making them useless)