How to stop a YouTube process running in Chrome?
You can open Task Manager, under Window on the Chrome.app menu bar, and see a list of all running processes inside Google Chrome.

You can see Task Name, Memory, CPU, Network and Process ID for each task running at the moment.

At the bottom of the Task Manager Window you can end the Task you selected by pressing End Process. 

[EDIT]

If you are on Windows the Task Manager window is accessible via the Shift + Esc shortcut.