How to close IntelliJ IDEA if it hung?

It's running as a java process so you can use

killall java -9 

(Warning! This command kills all java processes)

Source: Intellij support forum

You can list all processes with

ps aux

and you can search processes with

pgrep <process name>

Then you can kill the process you want with

sudo kill -KILL <pid>

You can read more about it here