Repositories are created on your local machine, and are then pushed to us, but at that time, it already exists. There's no way we can actively prevent you from putting binary files in a repository due to the nature of DVCS.
I think it is a way (at least) with mercurial (at least). Checking changeset files in the pretxnchangegroup.check hook it is possible to reject a push. But I do not know if bitbucket allows to provide server-side hooks for repositories. I do not know how to do this with git.