How to completely remove and disable Robokiller app from my iPhone?

If you're talking about this app:--> https://apps.apple.com/us/app/robokiller-spam-call-blocker/id1022831885 , use these instructions: Settings>General>iPhone Storage... scroll down to that app, tap and delete it.

For any further questions about that app, please contact their developer/support team here:--> https://support.robokiller.com/hc/en-us

Regards

Giulio