cakes... sounds awesome, but as for turning off migrants, that sounds kind of artificial to me
Personally, I can live with the break from reality. For those who can't, though, there is the option of not using it, which would leave things as they are. Then, when you suddenly decide to write an epic tale of
The Last Dwarf on Earth, or
Seven Dwarfs Who Got Stranded on an Island Or Something, etc., you can switch to Migrants: None and let the Fun ensue.
ETA: If an in-game justification is needed, of course, we could always hand-wave it as being the fortress's immigration policy. Maybe migrants have to send in applications or something before they come, and a fortress that's set to None is considered to simply decline all requests. /shrug.
ETA2: ...And that just gave me another possible idea. You guys ever played/ heard of a game called Tropico? One of the buildings you construct is an immigration office which, when fully (and competently) staffed, gives you control over who comes and goes. If we really prefer realism to metagaming, we could probably make it an in-game function ala Tropico: Just as the Bookkeeper allows you to know how much stuff you have, the Immigration Official allows you to set the skills/amounts of migrants, and embarking with a Dwarf with points in I.O. would allow you to take care of this from the get-go. Of course, this would use up skill points that would be otherwise allotted, which could make the Hermit scenario that much harder. This could be fun, although personally, I think the meta way is easier to deal with.