Removing hole-in-wall gas fire - what about the flue?

[Gabe]

We have a defunct hole-in-wall gas fire in our lounge that we want to remove, and create an alcove in its place - i.e. we no longer want a gas fire. My question is what to do with the existing flue - it goes up between two internal walls, through the loft, and vents out of the roof ridge. Can we simply seal it off at the bottom?

 

Thanks.