Hopefully, receiving unwanted calls or messages is not a problem for you, but unfortunately, it is for many. It can start as a minor annoyance, but quickly turn into a major probem if it isn't addressed quickly enough. No one should have to put up with this.
There are two ways to mitigate this annoyance/problem. One way is to simply avoid answering the call or replying to the message each time. The problem with this method is you are still subjected to knowing that they are calling and/or messaging you. The alternative (and much better, long-term solution) would be to add that person to a list of blocked callers.
Before you can add someone to the list of blocked callers, you will first need to create a Contact entry for the person, if you haven't already (you'll see why later). The easiest way to create a new contact would be to look at your call log ("Recents") or message history and create a new contact from there by tapping on the appropriate links. When you create a new contact, you may want to use a standard naming convention like first name = "Blocked" to make it easier to identify these contact entries later. This, of course, only applies to those who's name you don't know. If you want to block someone you know, you would want to use their actual name.
Now that we have created a new contact for the caller we want to block, we need to add that contact as a blocked caller. To do this, go to Settings > Phone > Blocked. From here, you will tap "Add New..." to add a caller to the list. This will open your list of contacts. Browse to the new contact you just created and add that contact to the blocked caller list.
With the person on the blocked caller list, traditional phone calls will go directly to voicemail, FaceTime calls will simply go unanswered and messages will show "delivered" to the sender. From your perspective, traditional and FaceTime phone calls never make it to you and are not listed in your call log. Although messages show as "delivered" to the sender, they never make it to your phone. True silence.