You'll want to find the pid (process id) of the running NodeBB process. Use ps aux | grep node to see all process that are node based.
Then kill it by pid.
e.g.
[email protected] ~/nothingtoseehere (master) $ ps aux | grep node
julian 11381 5.5 1.3 959772 53680 ? Ssl 18:25 0:00 /home/julian/.nvm/versions/node/v4.4.0/bin/node /home/julian/nothingtoseehere/loader.js
julian 11396 3.7 0.9 910404 37308 pts/5 Sl+ 18:25 0:00 node ./nodebb log
julian 11405 44.0 3.8 1303708 153892 ? Rl 18:25 0:05 /home/julian/.nvm/versions/node/v4.4.0/bin/node app.js
julian 11483 0.0 0.0 15192 2260 pts/6 S+ 18:25 0:00 grep --color=auto node
Looks like the NodeBB app is on pid 11405, and the loader is 11381. kill 11381 will kill both (as the app is a child of the loader)