Stream-sniping is the act of hijacking or derailing another user's live stream to either prevent them from completing a task or simply to harass that user.
If you were stream sniped, meaning someone intentionally joined your game to gain an unfair advantage by watching your stream, you could make a complaint by contacting the GameBeef support team through Discord. Provide them with the necessary details and evidence, such as video clips or timestamps, to support your claim. The GameBeef staff will review the situation and take appropriate action.
It's paramount that you gather all evidence such as visual aid that prove this incidents actually happened.