The American Ballet Theater yesterday cut off all payments for medical insurance for its dancers, who are locked out in a labor dispute. The cutoff began with the September payment, which was due last Friday. ABT acknowledged yesterday it had not made that payment and would not until the dispute was settled, but said none of the dancers was in danger of losing medical benefits because of a three-month grace period at Blue Cross and Blue Shield, the dancers' insurers.

The dancers' union, the American Guild of Musical Artists, called an emergency meeting for this morning to find a way to pay the insurers. The union's national executive secretary, Gene Boucher, termed the cutoff "inhumane."

ABT's general manager, Charles Dillingham, said ABT intends to pay all past-due funds for medical benefits when it reaches an agreement with the dancers.

ABT locked the dancers out seven weeks ago and has canceled its remaining 1982 performances, including those at the Kennedy Center.

New York State law prohibits the payment of unemployment insurance to people who are involved in a labor dispute for the first seven weeks of that dispute. Many dancers have taken loans from the union's relief fund to tide them over, while two are selling their blood to raise money and two are participating for payment in experiments that require the extraction of bone marrow, according to Frank Smith, spokesman for the dancers and himself a veteran ABT soloist. "ABT is trying to starve us out and some of us are starving," Smith said.

In Los Angeles, ABT's associate artistic director and board member Nora Kaye pledged her support to the dancers.

"Some of the dancers are really starving and I feel desperately sorry for them. This is wrong," said Kaye.