ignore .DS_Store

Remove existing files from the repository:

find . -name .DS_Store -print0 | xargs -0 git rm -f --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!'