{
  "detail": {
    "lastStatus": ["STOPPED"],
    "stoppedReason": [{
      "anything-but": {
        "prefix": ["Task stopped by user", "Scaling activity initiated"]
      }
    }]
  },
  "detail-type": ["ECS Task State Change"],
  "source": ["aws.ecs"]
}

This configuration works. There is also example in documentation under the Anything-but matching on prefixes section.

• https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-create-pattern-operators.html#eb-filtering-anything-but:~:text=Anything%2Dbut%20matching%20on%20prefixes