To remove Hiya from your iPhone:
1. Exit the app and long-press the app icon.
2. An "x" will appear in one corner. Tap the "x" to completely uninstall.
If you are still having issues after uninstalling Hiya you might have to reinstall the app and then try these steps:
1. Disable Hiya in the iPhone Settings (Settings->Phone->Call Blocking & Identification)
2. Force-quit Settings
3. Uninstall Hiya
4. Restart your device