Under Administration -> Device Capture you'll find all the in-progress device captures and there will be a cancel button by the ones that are still in progress. I don't know of a way to do them all at once using the GUI.
You could also try shutting down the appliance services and/or rebooting the appliance.
There are thousands in process - is there a way to issue a kill on ALL of them simultaneously?
From the SSH command line you can stop the services (commands assuming you're on CentOS 7):
Wait for the services to stop fully
When I got back into my appliance, the snmp captures appear to have stopped, but I was testing with only a couple running.