The previous owner of my house had a sprinkler system installed and had it run off the main water supply in the house instead of a separate meter. I had a new meter installed, so I do not have to pay sewage on my irrigation water. Now I have a pipe sticking through my foundation that has been cut off and capped on each end. It is not needed any more. If I remove this pipe, what is the best material and process to use to fill the hole in the foundation?
If it's not a huge pipe, hydraulic cement would probably do the trick. Hydraulic cement is designed to expand as it cures and is used for reparing foundation cracks.
You don't need anything structural - just something to cap the hole and be an approximate match to the rest of the concrete.
In that case you just needs some "patching" mixture - some ready mix post concrete springs to mind.
I'm assuming that the hole is horizontal - if so then just push enough mixture in to make it water tight and smooth it out to match the current surface. Repeat for the other end if necessary.