If .DS_Store was never added to your git repository, simply add it to your .gitignore file.
.gitignore
In your the root directory of your app and simply write
**/.DS_Store
In it. This will never allow the .DS_Store file to sneak in your git.
find . -name .DS_Store -print0 | xargs -0 git rm -f --ignore-unmatch
then commit and push the changes to remove the .DS_Store from your remote repo:
git commit -m "Remove .DS_Store from everywhere"
git push origin master
And now add .DS_Store to your .gitignore file, and then again commit and push with the 2 last pieces of code (git commit..., git push...)
If .DS_Store already committed:
find . -name .DS_Store -print0 | xargs -0 git rm --ignore-unmatch
To ignore them in all repository: (sometimes it named ._.DS_Store)
echo ".DS_Store" >> ~/.gitignore_global
echo "._.DS_Store" >> ~/.gitignore_global
echo "**/.DS_Store" >> ~/.gitignore_global
echo "**/._.DS_Store" >> ~/.gitignore_global
git config --global core.excludesfile ~/.gitignore_global