Modifiers for campaign relevance

[downer]

Another thread just reminded me that there is, as far as I can tell, no easy way to grade advantages and disadvantages by relevance to a given campaign. The problem is basically this:
What (except GM fiat) is to keep a player from picking a disadvantage that is possible, but from a campaign standpoint mostly or completely irrelevant? Say, a character has Thalassophobia. Sure, the world has oceans, but the player knows that the campaign is set in the middle of a major continent and centers around a landlocked kingdom. He knows that the GM has pages and pages of background on that kingdom, and won't let the characters leave it, unless they have a real good reason. So, Thalassophobia is essentially free points and does little to provide characterization.
By contrast, what to do if a player comes up with a nice background story, which includes a Social Regard, or some such. But that advantage is for a far away land. Technically, it still costs points, but won't help him much.

How do you deal with that? I can see some options:
1. Simply reduce the point cost of everything you consider irrelevant to 0. No bonus points for your Thalassophobia, no cost for your Social Regard. That seems a bit harsh and arbitrary, and leaves a large middle ground open.
2. Forbid all such things. Require that the players make characters who are tailor made to the setting, have a local history and local habits. Like 1., a little harsh and also leaves out the middle ground.
3. Add a percentage modifier. If so, how high should it be?
4. Use the Frequency of Appearance guidelines (maybe actually roll).