This is how i force exit or close WhatsApp from running (even background process) on my Nokia 5800 XpressMusic.
- Install NetQin Mobile Guard 3.0 for Symbian and run the application.
- Click on OS Scan and NetQin will display a report after scanning your phone.
- Go to "Auto-start apps" to disable WhatsApp from auto start whenever the phone restarts.
- Go to "Memory usage" to close currently running WhatsApp application.
- WhatsApp is now completely turned off.