Our chimney has two flues, one we use the other must have been for a woodstove in the basement but has since been covered up with a finished basement. When we burn in the fireplace we can smell the smoke downstairs and from what I've been told it's because air has to come into the house to replace the air going out the chimney and the path of least resistance is down the other flue so smoke comes back down and into the house. I think the easiest solution is just to seal off the flue from the top, is that right? If so how should I go about doing this?
You didn't mention what the opening was that the smell was entering the house with. If it is a thimble opening, you can block the opening with solid mineral wool insulation, cut to the size of the opening and fit snugly. This will stop the downward drafting. Add a thimble cap over the insulation and it will look fine. It also will stop cold air from entering the house through the flue.
You could also put a flat concrete patio block. cut to just larger that the flue opening, over the unused flue. You can secure it in place with some exterior masonry caulking. This will stop the drafting, but also make it possible to remove the cover if you decide to use the second flue for another purpose.
Yes, that is correct - sealing the top of the unused flue is best.
Many possible approaches - a stainless steel cap and mortaring a tile over the flue opening are two that come right to mind. If you're not comfortable working up on the chimney top, hiring a chimney sweep might be best.