How to disable git push when there are TODOs in code?

We are having an issue in our team and we have decided to check if there is a way or git command to reject git push where there are TODOs in the code. Any ideas? Thanks in advance.

EDIT: because of grep behaviour in if statement we needed to edit our script:

#!/bin/sh

for FILE in `git diff --name-only --cached`; do
    grep 'TODO' $FILE 2>&1 >/dev/null
    if [ $? -eq 0 ]; then
        echo $FILE ' contains TODO'
        exit 1
    fi
done
exit

Is not possible use pre-receive hooks in github, so we are using instead pre-commit hook in client side: http://git-scm.com/book/en/Customizing-Git-Git-Hooks#Client-Side-Hooks

Our pre-commit script (based on http://mark-story.com/posts/view/using-git-commit-hooks-to-prevent-stupid-mistakes) looks like:

#!/bin/sh

for FILE in `git diff-index -p -M --name-status HEAD -- | cut -c3-` ; do
    if [ "grep 'TODO' $FILE" ]
    then
        echo $FILE ' contains TODO'
        exit 1
    fi
done
exit

We have this script under our control version system, and create a symbolic link to it in .git/hooks

Thanks for the help :)