Make the game stop if the ball hits the bottom.
Click to select the Stage:

Select the Backdrops tab.
Now you can edit your backdrop. Select the line tool. Draw a thick line at the bottom:

Tip: Hold down the Shift key while drawing the line to make it straight.
Tip: To undo, click the Undo arrow:

Click to select the Ball sprite:

Click the Scripts tab, then add this script to make the ball stop if it hits the line color:
