This phone-shaped app should be on your Android's Home Screen. Tap the three-dot icon in the top-right corner of the Phone screen. Doing so prompts a drop-down menu. This option is at the bottom of the drop-down menu. It's below the "CALL SETTINGS" heading near the middle of the screen. Tap the text field below the "Add phone number" heading, then type in the number you want to block. This button is in the keypad at the bottom of the screen. Doing so will save the number to your Samsung's block list.

Summary:
Open your Android's Phone app. Tap ⋮. Tap Settings. Tap Block numbers. Enter a number. Tap Done.