Ask your phone company. Some offer call blocking free or for a small fee.
Or you can buy a call-blocker, an electronic box that plugs into the phone line between the phone and the wall. It also plugs into an electric outlet. It comes with instructions on how to program in the numbers you want blocked. Call Blockers are available at Best Buy and on Amazon.
They call and ask for my husband by first name. When I say he's not available and ask who's calling they hang up. I call the number back and I get a busy signal. I tried several times and the number doesn't work.