I have an old barn that is post & beam constructon with termite damage to some of the uprights. When it was built, the posts were just set 2 feet into the ground with cement poured around them for stability. Now termites have damaged several of these and I need to build plith blocks to fit in new uprights. My question is, how to get out the wood that remains in the ground after taking out the damaged post? There is a nice square 4x4 hole in the cement footer that the first 8 inches are clean, then ~ 18 inches of semi deterioratd post that won't readily come out. I don't want to build my plith blocks on top of rotten old wood, but would rather fill the old cavity with new cement and tie to the plith block with rebar. Any suggestions would be appreciated.

The deeper you go the more the wood should be rotted. So it should be a fairly easy task.
You can get a large power drill and a fairly aggressive paddle bit with an extension. Simply drill lots of holes into the remaining wood in the cement block and vacuum out the remaining debris.
However one thing comes to mind here.
When these posts were placed into the ground the cement was placed around them to hold them tight. Your task would be to set the new posts into the prior holes, which for all purposes will not fit if your using the same size post.
Cutting them down to fit into the hole will place a smaller foot print of the wood on the bearing soil or hopefully gravel base. This in turn will cause the post once the weight is place onto it to sink into the soil because of its smaller size.
My suggestion is to remove the cement pillar that holds the post into place and re-pour the entire footing. Do not forget to use plenty of gravel on the bottom so any water can drain away from the bottom of the post. Also dipping the post in a liquid asphalt before setting will help preserve the wood longer.