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.

From the SSH command line you can stop the services (commands assuming you're on CentOS 7):

tw_service_control --stop

Wait for the services to stop fully

tw_service_control --start

When I got back into my appliance, the snmp captures appear to have stopped, but I was testing with only a couple running.