An unidentified caller commenting on the air about radio station KPFK-FM's Wednesday coverage of the Iran- contra hearings used a vulgarity to describe members of the Reagan Administration, an apparent violation of new broadcast obscenity standards issued last month by the Federal Communications Commission. The nonprofit station has been under fire for airing an allegedly obscene program last year, and the FCC has referred the case to the Justice Department for possible criminal prosecution. Wednesday's caller was warned about using obscenities, and on-air hosts Peter Bergman and Alan Fong noted the Pacifica Foundation station's continuing problems with the FCC. According to acting program manager Blair Zarubick, the station does not have the technical means to delete comments by callers. The comment was made at 10:36 a.m. and was noted in the station's program log, Zarubick said.