Remove existing files from the repository:
find . -name .DS_Store -print0 | xargs -0 git rm --ignore-unmatch
Add the line
.DS_Store
to the file .gitignore, which can be found at the top level of your repository (or created if it isn't there already). Then
git add .gitignore
git commit -m '.DS_Store banished!'
Or specify a global exclusion list
git config --global core.excludesfile ~/.gitignore
echo .DS_Store >> ~/.gitignore