cd /... into your repository, then...
find . -name .DS_Store -print0 | xargs -0 git rm -f --ignore-unmatch
.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). You can do this easily with this command in the top directory
echo .DS_Store >> .gitignore
git add .gitignore
git commit -m '.DS_Store banished!'