How do I remove a stack from the dock? I pressed control - options, but then I couldn't find the "remove from dock" option, only "open in finder"

Just drag the icon away from the dock, release the mouse button when stack icon becomes a cloud

Since this account is controlled by the Parental Controls, I would guess that the option to prevent the Dock from being modified as been set, though why you were able to inadvertently add folders to the Dock I don't know. If so, then you'll need to log into the system as a user with administrator privileges, uncheck that option, then go back to the account whose Dock you wish to modify and then remove the folders.

Regards.

Is the anything special about the Stack?

I've never seen Move to Finder, or did you mean Show in Finder?

Try holding down command and drag it from the Dock. Not a lot of confidence in that, but give it a try.

Using Barney's suggestion, make sure you drag it until you see a puff of smoke.