When I run git reset --hard HEAD, it's supposed to reset to a pristine version of what you pulled, as I understand it. Unfortunately, it leaves files lying around, as a git status shows a big list of untracked files.

How do you tell git "Just bring it back to EXACTLY what was in the last pull, nothing more, nothing less"?

You have to use git clean -f -d to get rid of untracked files and directories in your working copy. You can add -x to also remove ignored files, more info on that in this excellent SO answer.

If you need to reset an entire repository with submodules to the state on master, run this script:

git fetch origin master
git checkout --force -B master origin/master
git reset --hard
git clean -fdx
git submodule update --init --recursive --force
git submodule foreach git fetch
git submodule foreach git checkout --force -B master origin/master
git submodule foreach git reset --hard
git submodule foreach git clean -fdx