There is no ELSE statement as it is, but you can use the regular events.
In our case,
ball.x < 0 //Left boundary
... Ball => Destroy
ball.x > 300 //Right boundary, you can also use "layoutwidth" or "windowwidth"
... Ball => Destroy
ball.y < 0 //Top boundary
... Ball => Destroy
ball.y > 600 //Bottom boundary, you can also use "layoutheight" or "windowheight"
... Ball => Destroy
This way, if the ball is out of boundaries, we destroy it.
Layoutheight/layoutwidth and windowwidth/windowheight are system expressions.