If you have an Android phone, that capability should already be available without an additional app.  On my phone, an LG Optimus, I go into Call Settings.  On the Call Settings screen, I select "Call Reject" and then set the option to reject calls based on a call reject list.  I then add the number, from the Call Log, to the list.  Once added, any calls originating from that number are rejected.

My reject list has grown quite large.  I think these robo-callers just dial a series of random numbers until a connection is made.