That just adds an access-control entry to the security descriptor of the file. So can simply delete the same.
This may require reconfiguration to show up security descriptors in the first place; I forget, since I have always turned off the Windows options that hide the underlying OS features.
Right-click on the folder in Explorer. Bring up the folder Properties. Select the Security tab. There will be a list of who can do what to the file.
In the OP's scenario, he will have added an entry that says "some particular limited user has (I would guess) full control access to this folder".
Remove that entry via "Edit".
Be careful. You can do a lot of damage by removing access that needs to be there. You might, for example, think "the SYSTEM has no right to see my files". Well, maybe. But on the other hand you perhaps just destroyed the ability of the operating system to operate. Ditto with Administrators. "Users" are not always people, they're also code executing in the OS.