The state has revoked the credential of a San Francisco teacher more than 18 months after she was fired for terrorizing her young charges with threats of physical abuse. The Commission on Teacher Credentialing determined that Joyce Ferrer had stuffed children into a garbage can and banged on the lid while she was a first-grade bilingual teacher at Bret Harte Elementary School. She also pushed children’s heads toward the toilet bowl while warning them she would flush them down, the commission found. And Ferrer, 43, a 16-year teaching veteran, also threatened to make children spend the night alone in the classroom with “rats and spiders” and told one talkative boy she would cut out his tongue with scissors and sew his mouth shut. Ferrer has denied all allegations. San Francisco officials found Ferrer “unfit to teach” in April, 1987, but the procedure to revoke a teaching credential is a lengthy one and it was not until Nov. 4 that the action was taken.