um, why not just change to whatever gear you want and then never shift gears again?
you effectively have a single speed if you never shift gears.
that's essentially what you're describing. if you remove the derailers you have to make sure that your chainline is dead-on straight, so the chain won't fall off. and this setup has several drawbacks, like you won't be able to replace your cog with a bigger/smaller one, you have several unused sprockets, you might not be able to get a straight chainline without adding/removing spacers/redishing the rear wheel, etc. IMHO, just get a new hub and cog.