How so I delete a spam call on iPhon 8?

I know how, in theory, to delete a spam call left on my iPhon 8, but when I slide the call to the left, as I press on the red Delete message that appears it does not delete the call at all. It frustratinly brings up the options to Call, Message, Add to Existing Contact, and Create New Contact. It does not delete the call or give me an option to delete the call at that point. What do I have to do to stop making this happen? Otherwise, I have to turn the phone completely off, then restart it again, and go back to the list of callers and try over, and over to slide the spam caller to the left and try again to press on the red delete option, which 9 times out of 10 it goes on NOT delete the call but brings up the options again, and again, and again, until I somehow get lucky and finially the red delete option "takes" and the call is deleted. Why do they make it so difficult to DELETE a spam call - or any call Ifor that matter that I don't want to keep? What do I do? I already know how to block a caller. But then how do I go back and DELETE the call?