If you plug it, you might end up overflowing it to the floor.
If you cup is filling that fast it is usually the water flow. Too fast of flow and the skimmer does not have time to process the bubbles and pushes it straight through.
Another culprit is "new skimmer syndrome". You can remove the cup and leave the skimmer to run. Throw the cup in your tank or sump. After a few days your skimmer should develop a "film" you can't see. Like a biological film of some sort. Then try again.
Lastly it could be something you are dosing. Skimmers will over react to certain things.
I hope this gives you a start.
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