Projects → Survey menu → Project properties
This feature is primarily used to protect content: It deactivates the context menu in the entire questionnaire.
Keyboard commands however will not be suppressed.
Using a question of the type Text and image, you can realize the same behavior for a particular questionnaire page. The feature can also be used to prevent the usage of the browser’s Back button. For this the survey must be opened in a pop-up window, so that there is no menu bar.