I believe it is pretty futile for an open-source project to “prohibit” third-party binary distributions of their software (regardless it is distro packaging, AppImage, Flatpak, Snaps or others), the best you can do is to mitigate the harm from them via better communication including but limited to:
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Documentation and issue templates
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Requiring the downstream publisher to:
- Specifically marking the distribution as UNOFFICIAL
- Drop/replace upstream support contacts
- Do whatever patching to the distribution so that the user will be forced to acknowledge that this is not an upstream-supported distribution (e.g. Inform users with custom dialogs )
I believe most (active) snap publishers will be willing to comply with these changes.
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As a last resort, request the distribution platform(e.g. the Snap Store) to:
- Mark the snap distribution as unlisted (to not appear in search results or recommendation).
- Drop the distribution when the publisher cannot address the problem within a reasonable time-frame.