it happens... like alot, nearly every jam ive joined with a prize has it happen. limiting the voting to submitters and having a large pool of submissions help negate the 1star spams... but still discouraging to see it happen, really wish ITCH.IO has more effective ways to moderate jams. when i hosted B1T Jam, ive partnered up with Jamlytics to help moderate and catch cheaters... helps reasure any participants that the jam is safe and fair.
winning/losing a jam is more fun when its fair, having cheaters around ruins the experience.