gitignore

# remove any existing files from the repo, skipping over ones not in repo
find . -name .DS_Store -print0 | xargs -0 git rm --ignore-unmatch

# specify a global exclusion list
git config --global core.excludesfile ~/.gitignore

# adding .DS_Store to that list
echo .DS_Store >> ~/.gitignore

git add .gitignore
git commit -m '.DS_Store banished!'