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I guess you can disqualify any entries that don't follow your rules, but I don't think that a requirement to release your source code to comply with the rules of a jam would legally compel you to do so. A requirement to allow your content to be hosted on some site would seem to me to be a different prospect altogether - are you asking people to sign over distribution rights for their game to you in perpetuity? That kind of stuff is usually spelled out in more detail (e.g. the itch.io terms), and if you're expecting to hold people to a contract based on them entering your jam, you should probably do the same.