So, unless you license otherwise, your work is implicitly copyright and you can sue anyone who attempts to distribute or sell your work without permission, except in cases of purely academic use. You can add a copyright notice at the bottom of your repository README.md file.
Example: Copyright {year}, {owner's legal name}. All rights reserved.
Source: https://www.termsfeed.com/blog/sample-copyright-notices/.
Also, you can make your repository private. You can give Lee Reilly (the jam host) permission to access your repository, but judges do not need to see your code to judge your game. Your submission can be an itch.io game page, which hosts info on your game and distribution packages, like .exe, .rpm, .deb. That way your source code is not immediately accessible.