It is unlawful to continually, intimidate or harass an individual over and over again.
If you keep getting letters but have advised your distributor of your refusal to have an AMI installed, try using this bit of legislation, write to your distributor & complain to the ACCC
“ACCC Sales practices – Consumer Affairs Victoria
Section 07. Harassment and coercion
Harassment and coercion
It is unlawful to use physical force, coerce or unduly harass someone about the supply of, or payment for,
goods or services.
ACL reference: section 50
What is harassment and coercion?
It is unlawful to use physical force, coercion or undue harassment in connection with the:
-; supply or possible supply of goods or services
-; payment for goods or services
-; sale or grant, or the possible sale or grant, of an interest in land, or
-; payment for an interest in land.
Undue harassment means unnecessary or excessive contact or communication with a person, to the point where that person feels intimidated, tired or demoralised.
Coercion involves force (actual or threatened) that restricts another person’s choice or freedom to act. Unlike harassment, there is no requirement for behaviour to be repetitive in order to amount to coercion.
Legal reference: ACCC v Maritime Union of Australia  FCA 1549”
Similar advise was posted earlier on SSMA and the link below will take you to this previous post that also includes a pre drafted letter of Undue Harassment.