I assume the RoboKiller app works with iOS's Call Blocking & Identification feature.
First, disable the feature, go to:
Settings -> Phone -> Call Blocking & Identification.
Find the RoboKiller app and disable it.
I do not know the RoboKiller app, but if there is an option in the app where you can turn it off, do so. If you do NOT want to use the RoboKiller app anymore, delete it.
If you can not find the app on your home screen:
If you cannot find the app, it may be because it is in the App Library. Swipe as far to the right as you can until you get to the App Library. Find the RoboKiller app, long press and drag it back to the home screen. You can also delete it directly inside the App Library. Long press and select Delete.
If you are not allowed to delete an app:
If you do NOT have the option to delete the app, it may sound like you have a Screen Time restriction enabled, go to:
Settings -> Screen Time -> Content & Privacy Restrictions -> iTunes & App Store Purchases -> Deletings Apps.
It must be set to Allow before you can delete apps.