Quite simply you don’t announce it at all and you remove it from the station CIS▸ . Sure, the odd person might get on, but the majority won’t. As long as more people leave than join then you’ve improved the situation. The crew also have the ability to easily put a ‘not in service’ message or blank the external displays on the IETs▸ .
Just working that one through using Phil's example - summer Sunday afternoon at Exeter St Davids, platforms busy, almost certainly soaking up a number of long distance cancellations given that it's a summer Sunday and staff don't want to work.......train pulls in, a few people get off, already delayed customers ask platform staff ("Is that a London train?") - what are they supposed to say in reply? "Yes, but you can't get on"?
Given a busy platform, with the best part of an hour to wait for the next train, how do you think that'd play out?