Unless you literally own me, how would you possibly prevent me from doing new work without giving it to you? Even if I were under an employment contract that assigns all my intellectual property to you, as a free person I would be free to quit that job and cease giving you any further IP.

For the record that my suspicion is that bug fixes to open code are very likely not copyrightable due to the principle of convergence, though I am not a lawyer and to my knowledge this issue has never been tested in court. New features would very much be copyrightable.