Hello.
Submissions can't be stopped or started manually. They are always being calculated, so if yours looks stuck at 0%, it is most likely just waiting in the calculation queue. Occasional load spikes make submissions take longer than usual. You don't need to worry. Every submission will be calculated, so yours will get its turn.
If you'd rather not wait, you can remove it. There's a Delete button in the Candidates tab. I understand it's frustrating that the same isn't possible from the Checking tab, so we're adding a Delete button there soon.
Thanks for the report.
