Samsung GALAXY S8 User Manual page 142

Block a Number
By adding a caller to your Block list, future calls from this number are
sent directly to your voicemail, and messages are not received.
1. From a Home screen, tap
2. Tap Recents.
3. Tap the caller you want to add to the Block list.
4. Tap Details > More options > Block contact. Confirm when
prompted.
You can also modify the Block list in Settings:
► From a Home screen, tap
Block numbers.
Apps
Phone.
Phone > More options > Settings >
132