Galaxy Nexus - How do I start adding strangers' numbers to "auto-reject"?
If it's the same people ringing you regularly what you could do is:


Add them as a contact, under 'Ignore1', 'Ignore2' etc. whenever they
ring 
Go to Contacts -> Select 'Ignore1' -> Options (or action menu) -> Select 'All calls to voicemail'


Or if you don't like this method then you could try:


Mr. Number: Text, Call &amp; Block
(free)
Extreme Call Blocker Droid  (paid)