I have an old toilet in the unfinished part of my basement that I've never used. The water to it is turned off. I have no intention to ever use it, but I don't know if I need to do anything to ensure sewer fumes don't come through it. (I haven't had any issues but have heard this could become an issue.) Do I need to remove it and plug the hole in the floor, or is there something I can do while leaving the toilet intact (i.e. plug the toilet itself). It's an old toilet so I don't mind if I render it un-useable.

Pour a little concrete in the bottom, put a piece of plastic over the hole in th bottom and put some sand in it, fill the hole with foam or just pour a quart of water in it every month.