invalidate

If administrators find out that students cheated on a test, the school might invalidate their class grades. To invalidate means to cancel something or make it void, as if it never happened.

In invalidate you see the word valid which means true or correct. When you invalidate something you are making it less true, less official, or less correct. If you buy something that doesn't work properly and then try to fix it yourself, you invalidate the warranty. If you have a wacky theory that cars grow from trees, your teacher might invalidate your theory by taking you on a tour of an auto factory.