There are a number of ways to block a call from the app. Note that all calls on your block list will be sent to voicemail.
If you received a call with a Trust Score - you will have seen this either when the call rang or when looking the number up - then you can change your Trust Score to stop this number ringing again. Changing your Trust Score to exclude this call. For example, if this call had Trust Score 3, then choose a Trust level of 4 or 5 using the Trust Score Slider. If the call had a category (for example, charity), you can select to block all calls from charities in the custom settings.