I'm not able to kill another user's background process using ActivityManager.killBackgroundProcesses.
Code used is:
 ActivityManager am = (ActivityManager)getSystemService(ACTIVITY_SERVICE);
 for (RunningAppProcessInfo pid : am.getRunningAppProcesses())
 {
     if (pid.processName.equals("com.example.sample"))
        am.killBackgroundProcesses("com.example.sample");
 }
Where com.example.sample is the package of the background process which I want to delete.
That process is still running in the background after this completes.