Return to your course front page and click on the link to your quiz.

Press the Update this Quiz button in the top left-hand corner of the page:

Scroll down the Editing quiz page until you find the Attempts box. Change Adaptive mode from Yes to No. This turns off the Submit button.

If you only want students to have just one attempt at your quiz, you can select 1 attempt from the Attempts allowed drop-down menu:

Scroll down to the bottom of the page and press the Save and display button. If you click on the Preview tab, you'll see that the Submit button has now disappeared:

That's all there is to it!