When a shovel drops, pick it up and a timer for it will appear. Return to the main menu, then continue the same level. The timer will disappear and you will have indestructible walls for the rest of the level. Note: Nukers (can go over the wall) and Zappers (can break wall) can still get through it. Also, you can attempt this with the timer power-up to get unlimited an power-up timer.
Power-ups hint: Every power-up collected also gives points at the end of the level. You can use the points to buy ne stuff. Make sure you collect all power-ups as they are found, even if they are not needed at that moment.