When and who
The main ONE Pass holder or an EA acting for the holder cancels when the dependant no longer intends to stay in Singapore. The detailed rule is within the week before departure. A separate cancellation is unnecessary when the dependant becomes a PR or the main ONE Pass is cancelled: family passes cancel automatically with it and cannot be reinstated.
How to cancel and remain lawfully until departure
Use EP eService through myMOM Portal; cancellation is immediate. If the dependant is still in Singapore, request STVP for up to 90 days from application. MOM emails it to the requestor. Show STVP with a valid passport on departure. Leave before STVP expiry; destroy the cancelled card by cutting it in half and discarding it. Overstay can attract a fine.
Official source
Written independently from official MOM source material.