@Carissa3 To start, open the Phone app. Tap the 3-dot menu in the top right corner, and from the dropdown menu that appears select Call Settings > Call reject > Reject calls from. Select the Plus icon, and then add the number you'd like to block from the call log or your contacts.
The wording might change between phone and Android versions.
Edit* For the S4 see here
A video walk through can be viewed here.