You can kill the ssh process like others with

kill -9 $PID

You can find ssh session process ID with

pgrep ssh

You can send a kill signal to the ssh process once you know the process ID of that tunnel task.

kill -9 $PID

You should search for the ssh session process ID amongst the others having ssh in it (sshd, pgrep, the tunnel process and perhaps several others.)

pgrep ssh

Choose carefully which process you kill.