Mich. Comp. Laws § 500.2078

Sec. 2078. Except as contained in the policy and the usual agreement for other insurance, no insurance company, insurer, corporation, partnership, or individual shall make any contract or agreement with any person insured or to be insured that the whole or any part of any insurance which is subject to the provisions of chapter 26 of this code (fire and inland marine rates), shall be placed by any particular corporation, partnership, or individual, or be written by any particular company, insurer, agent or any group of companies, insurers or agents. Any contracts made in contravention of this section, shall be null and void.