Members of the Clwyd North Division are against the changes at Glan Clwyd Hospital, Bodelwyddan.
These could see clinical staff transferred to Ysbyty Gwynedd and Wrexham Maelor hospitals from 6 April.
Betsi Cadwaladr University Health Board said it was "disappointed" by the vote.
Doctors, who held the vote after discussing the plans at a meeting on Monday, said the recent decision to withdraw consultant-led obstetric and gynaecology services at the hospital for 18 months has left them feeling demeaned.
Dr Richard Lewis, secretary of the Welsh British Medical Association, said that following a number of high-profile resignations at the board two years ago after a damning report on management failings, communications between the board and the wards remained poor.
"The situation in terms of the board's ability to communicate with its frontline staff are as bad if not worse than ever," he told BBC Wales.
"That gap seems not be closing; in fact on this particular issue it seems to be widening, which is a real worry."
Health board executive medical director Prof Matthew Makin said: "We fully acknowledge the strength of feeling regarding the board's decision from our clinicians but maintain that our primary motivation for making this pre-emptive urgent interim change is to protect the safety of mothers and babies."
Meanwhile, AMs in the Welsh assembly agreed a motion on Wednesday calling on the health board to "ensure the continuity of consultant-led maternity and other women's clinical services at each of the three district general hospitals in north Wales".
Conservative health spokesman Darren Millar, AM for Clwyd West, said: "I welcome the fact that not a single Assembly Member voted against our motion to protect maternity services from the threat of downgrading."
Doctors unhappy at proposals to close consultant-led obstetrics and gynaecology services at a north Wales hospital have passed a vote of no confidence in the health board.