There are a few methods to block a number, using the CallDetector app for Android.

You can access the Block List view on the Block tab and manually enter a valid 10-digit number, then select the Block button.  A pop-up message will follow, confirming that you would like to add the number to the Block List.

OR

You can go to the Activity tab, tap on the number you'd like to block, and select the Block icon on the detail view.