If you're serious about stopping the telemarketers, ask for their company and the full name of the caller if it's a non-electronic voice. Tell them, after that, that you will go to court if they call again. This puts the onus on the caller to deal with the situation and ensure you aren't called again. Just track the names, dates & times of calls on a notepad.
If it's a non-electronic voice with keypad entry, spam keys that aren't being asked for. (This also works with support hotlines) - The # and * keys are usually good ones to hammer since they aren't often used. This generally forces them to pick up the phone and talk to you in person (it can also be very annoying to them, as it harasses a more limited staff). After this, you can take the steps in paragraph 1.