How are you trying to klll it? The overlord API, or using the console? Have you tried both?
If nothing at all works, I suppose you could get the task ID in druid_tasks in the metadata database, shut down the middleManager, remove the row, and restart. However, I’ve never done this and it would be a hack, not a “recommended” method. I’d also back up the metadata database before making any manual changes, if you try this.