Step 1. Create an empty list called "Blocked Callers" in the Address Book Section.
Step 2. Select the newly created list and then add the number(s) that you wish to block the calls from and save it.
Step 3. Create an app called "Call blocker" (You can use any name) in the App bazaar section.
Step 4. In the newly created app (call flow), use the "Who's Calling" applet and choose "Blocked Callers" as the list.
Step 5. Now, ask Exotel to "hangup" calls if the caller belongs to the "Blocked Callers" list. Else, "Transfer" the call to your usual flow.
Step 6. Under the ExoPhones section, you need to assign your ExoPhone to this newly created app in order to do the checking first.
Now, as and when you find prank or spam callers, add their number to the "Blocked Callers" list from your address book page and then the call will be disconnected when a call is received from those numbers.