It was easy on bitbucket. if I dont want my repo to be forked. I can disable it. Only people I have invite can work on repo as long as they have permission on repo.
I'm not aware of any way to do this in GitHub, and frankly I don't see any value in Bitbucket letting you "disable" forking, either.
Bitbucket may let you disable the fork button, but anybody who can clone your repository can "fork" it by simply pushing it up to a different remote:
git clone git@bitbucket.org:alix/some-repo.git
git remote set-url --push origin git@bitbucket.org:chris/some-repo.git
git push origin master