When we run application in DEV mode (using command activator run) the process normally will be killed when terminal is closed. It's pretty easy since PID is started/closed automatically and therefore we do not care about it at all.
Once we start application in PROD mode there is a file RUNNING_PID is created (./target/universal/stage/RUNNING_PID). There is a command (since version 2.4) in activator (stopProd) which will close PID
$ activator stopProd
Alternatively you can just kill process 'manually'
$ kill $(cat target/universal/stage/RUNNING_PID)