I always answer my phone even if I know it's going to be a guy selling something. I make it a game trying to keep them on the line and waste their time. At the end I tell them that's what I am doing. They make less calls and bother less people that way.
For scamers that tell me the IRS wants me, I just laugh at them and tell them to send them on over. Then I tell them they really need to find a different job because the scam business is really not going to work for them, they are so bad at it. Funny, the number of calls I get has gone down.