There is always the possibility of foreign material entering the ground tube stake when it is not in use; however, we have never received a customer complaint about it.
The tip of the stake has a hole for moisture to drain, and it would take quite a bit of material to "pile up" in the tube to make the guide inoperative.
If you ever want to pull the stake out of the ground to clean it, you could use a claw hammer to get under the top flange of the stake to help lift it out of the ground.
We don't offer any type of plug for the opening; however, one could likely easily be obtained by using a cork. The opening diameter is 1/2 inch. Local hardware or home centers likely carry such items, which are usually available in either cork or rubber material.