Drastic measures to be sure but:
1. Do as others say. Do not use any credit. Cut up any/all credit cards you may have.
2. Opt out of all pre-approval mailings. There is an address you can write to. Let me know if you need it. I have a couple laying around here.
3. Put a credit freeze on your credit file with all 3 credit reporting agencies.
4. Pay off all credit cards, car loans, mortgages, etc and close all accounts.
In today's credit fueled world, just about every creditor in their terms & conditions state they have a right to and will share your information with related AND 3rd parties.
I wish you well.