Run ps aux | grep nodejs, find the PID of the process you're looking for, then run kill starting with SIGTERM (kill -15 25239). If that doesn't work then use SIGKILL instead, replacing -15 with -9.
—- update —-
It has been raised that this does not address killing a single node process and instead kills EVERY node process. If this is desired pkill is your tool, otherwise use one of the other accepted answers