If you tick 'Separate process' in your render Preferences, you can go into the Task Manager (Activity Monitor in OS X) and 'End Task' ('Force Quit') on the FFRender process without stopping Poser.

In Poser Pro, Render in Background also uses a separate process, whether or not you have the option ticked in Preferences.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)