You can delete the user, but you could also set the attribute 'Active' on false. The user will remain in the database, but has no rights to login in the app. Of course you need to delete/inactive the user after completing the survey.
Shouldn't be a problem to delete the user. Create a microflow that commits the survey fields and then deletes the current user. Then, use the microflow as a custom save button - put a microflow button on the webform with the survey, call the microflow you've created with that button, change the text and icon of the microflow button to match the standard save button and delete the standard save button on your form.
Only thing I am not sure of is what will happen to the 'created' attribute of the survey entity as you've just deleted the creator - warrants some testing.
Thank you, but is it possible to delete a user automatically when he/she logouts the first time? Maybe a microflow?