I take it that they are not using the same number each time. That is how a lot of them work. they know that people will be blocking certain number, even areacodes and prefixes, so they constantly change.
If this is what you are experiencing, then the best way to deal with it is by using an automated attendant. Because, in most cases, a machine is calling, it will not be able to respond. Even a simple set-up that gives out some information....
"Thank you for calling Bob's Auto Supply, we're located at 1234 Main street, anytown and are open from 9am to 6pm Monday to Friday...Press one to speak to the receptionist"
That will screen out any "non human" callers.