Israeli police have promised to stop impersonating journalists as a means of apprehending Arab protesters, the Foreign Ministry said. Police Commissioner David Krauss' order applies to all cases except as a last resort during hijackings or hostage-takings, and then only with his express approval, spokesman Alon Liel said. The Jerusalem Journalists Assn. had threatened to begin putting "Police on Duty" signs on reporters' cars unless the police stopped using the tactic, which journalists say endangers their lives in the occupied territories. The pledge followed controversy over two plainclothes officers with "Press" signs on their car who were filmed arresting Palestinian teen-agers in East Jerusalem last week.