While a typical user sees a reduction of as much as 90% in the amount of spam and robocalls received in the first 30 days, that still leaves another 10% that may get through. And yes, there are things that you can do to improve the service not only for yourself, but also the entire RoboKiller community.
If the offending caller was identified by RoboKiller, go to the RoboKiller Spam Box (aka the home screen), scroll to the entry for the caller and tap "Block." Doing this does two things: first, it adds that number to your personal block list and second, it tells us that we got it wrong, so that we can improve our spam detection in the future.
If the caller was not identified, you can go to the "Block" tab in the app and add the number manually, which will have the same effect as tapping the "Block" button.