Using Tasker to block a specific number
Per this link you can create a profile that defaults to phone silence and changes the ringer to not silent when someone you want to take a call from or is not a person you blocked and then when someone you have blocked calls it will answer the call and hangup.

Here is a summary


Create new profile Allow Calls
Add context --> State --> Incoming Call


Specify who to allow calls from (ex. C:FAV (for favorites) or !)
"!" reverses logic (such as is not contact) and 0 can be used for anyone calling from a restricted or unknown number

Add Task --> Audio --> Ringer Volume


select ringer level (7 is highest)

Add Exit Task --> Audio --> Ringer Volume 


select ringer level 0 (to silence the ringer once the phone call is done)

Create new profile Blocked calls
Add context --> State --> Incoming Call


Specify who to block (ex. !C:FAV (is not on favorites)  "0")

Add Task --> Phone --> Take Call
Add Task --> Phone --> End Call 


This should allow your phone to ring when a favorite is calling and pickup and hang up on someone you wish not to talk to without you even knowing.