Stung by losses under the federal health law, major insurers are seeking to sharply limit how policies are sold to individuals in ways that consumer advocates say seem to discriminate against the sickest and could hold down future enrollment.

In recent days Anthem, Aetna and Cigna, all among the top five health insurers, told brokers they will stop paying them sales commissions to sign up most customers who qualify for new coverage outside the normal enrollment period, according to the companies and broker documents.