Removing Old Cast Iron Clean-outs

I am replacing a subfloor in a bedroom in an old home. There are a couple of cast iron clean-outs that stick out of the ground in that room that the owner wants removed. He said that they are for a drain that is not used anymore.

What's the best way to remove them and to cover the opening?

Caulk in a c.i. flush plug.

What size are they and why aren't thy usable any more?

plugs

There are cleanouts and there are drains, but the two are not interchangeable. Make sure you know which you have before you discontinue them. There are several ways to terminate them starting with insertable expansion rubber plugs that can be cemented over.

They want these cleanouts cut off to below the level of the new subfloor. My helper says he has dealt with this before by using a sledgehammer to bust them off before plugging them. I thought there might be a more elegant approach. Would a sawzall with hacksaw blade work?

I think they are 3", but I'll take a photo of them next week when I start the job. The owner says they are not being used anymore because that drain has been re-routed to the exterior of the building and has new cleanouts.

I guess I am not totally understanding the situation since I can't see what you are talking about...so maybe none of the suggestions I am about to make will work for you, but can't you just undo the mj clamps holding that section with the cleanout and just cap it off below the subfloor? Or if that is not possible, since you are exposing the joists by removing the subfloor, can you not rent a snap-cutter and remove the sections and replace them with pipe or a cap? If you can't get a snap-cutter in there, you can always try a recip-saw with a special carbide blade, or even bash the pipe with a hammer and grind down the rough spots with a grinder (my least favourite approach), or even make a clean cut with a grinder if you had to.