Plastic happens to be the natural enemy of magic.
In "Saving WizTech, Part 1", WizTech is condemned after the mysterious infestation of plastic balls throughout the whole school, as it disabled any usage of magic. While Justin was occupying the school in order to find what caused the infestation, he finds out that Ronald Longcaped Sr. was the one capable of manifesting Wiztech with plastic balls (in which his plan was just throwing the ultimate birthday bash for himself). Justin attempted to use his magic to capture him but because of the plastic balls, Ronald Longcaped Sr. got away. Justin later gets the executives of Wiz World Industries to remove the plastic balls, in exchange for their products being endorsed at the school.
Wizards cannot get their magic through plastic, either. In "Night at the Lazerama", plastic is once again considered a hazard when Justin and Juliet get trapped in the mummy exhibit, and Justin is unable to use magic to get them out because of the plastic wall display.
In "The Good, The Bad, and The Alex", when Alex supposedly joins Stevie in her quest to overthrow the wizard competition, they conjure up a wall made of plastic bricks to prevent Justin and Harper from breaking through the magic portal to stop them.
Plastic is not fully impervious to magic though, in Saving WizTech, Part 1 when Professor Crumbs appears he brings several plastic balls with him, the mummy's magic was shown to be able to work through plastic in "Night at the Lazerama" suggesting that it might be incredibly powerful, or it works in a different way. In "The Good, the Bad and the Alex", both Alex and Stevie are able to summon a wall made of plastic bricks. Plastic seems to bend when magic hits it. (Night at the Lazerama)
- Although in the show there is no known reason why plastic is a magic repellent but it may be because the producers and writers of the show, namely Andrew Tsao, took many ideas from Sabrina the Teenage Witch in which plastics are also magic repellents, as well as other similarities.