Which RoboKiller app do you have? If it's the one by TelTech Systems, Inc, the Play Store has a comment by LC who stated the same issue. He, however, laments that app makers that 'do not provide a way to make contact with them' shouldn't be allowed. On the Play Store app page there IS developer contact info (website/email/adress) just below his post! Also, in the App Store, the app has a link to the developer web site where it states:
IMPORTANT: Once you've canceled your subscription, you'll need to deactivate the service on your phone. To do so, please visit www.deactivatemyphone.com. This tool is available 24/7 to ensure RoboKiller has been fully deactivated from your device.