I've never had a problem telling people that I'm not interested in their services. If they continue to call, block their number. If that doesn't work (or you can't) and they refuse to remove you from their list, add your number to a Do Not Call list. If they violate that, report them.
When all else fails, request their information (name, contact information, phone number, emails etc), and once you have all the information state clearly that you will be passing the information to the local police if they continue harrassment. That will surely get them to leave you alone.
I've not had anyone continue to call me after I tell them I'm not interested in their services, but there's a first time for everything I guess.
If you really want to go to the next level, Get their info, SEO the heck of out it, and post the bad experiences for everyone to see. Even better if you can get a recording of the call and post that to YouTube and the BBB also.