Unless you placed a veto prior to June 2003 you cannot prevent someone from contacting you. Adopted people and birth parents have the same limits and protections as everyone in the community. A person can only apply to restrict contact in violent or other extenuating circumstances.
If you do not want to be contacted you could write to your state government department and request a letter to that affect be placed on your file. This may not prevent contact but will inform the searching person of your wishes.
For other states click here, follow the links, contact the relevant government department to obtain additional information.