My personal motto is "I have no mottos"... ehem, I mean, my personal motto is "There are no impossible things but unable men". So the answer is, sure you can. Here comes one of my long mileage posts. I'm sure you must be familiar with all this, but, hey, maybe not everybody is, so forgive me if the explanation is too simple.
First option: a bike gearbox is know as a sequential gearbox. It follows the same principles of the gearbox of an F1 car. Here is the pic:
The gears without any marks are fixed to the axle, while the gears marked with an X are free to spin. It doesn't take too much time to figure that there are no two coupled gears that can transmit power: if one gear is fixed (I'll call it a "locker"), the one that meshes with it is free to spin (I'll call it a "spinner"). The gearbox is in neutral.
Now, the locked gears have "stumps" and the spinner gears have "holes" in them. If you moved the locked gear against its neighbour, both become interlocked, and then you have selected a gear. The "spinner" gear is locked to the axle through its "locker" neighbour.
It's easy to deduce that
if you cut the stumps or fill the holes (pick one), then the gear will not lock, so you will have one less gear to use. Another possibility (possibility
1A) is to elliminate (remove) the spinner gear for first gear, that is, the one marked with an X on the lower right extreme of the first picture.
There is a
second option: the selector forks are what move the locked gears and cause them to interlock with the spinner gears. They are like that:
It's easy to "see" that those forks, moving left or right is what cause the gears to "engage", isn't it? This is made through another device called a shift drum or ratchet drum. Here you have how it looks:
The little stumps on top of the forks are what allows them to be moved by the shift drum, through the slots that are cut on it. So, if you eliminate one of them, you'll have one less shift gear. You can also modify the shift drum, but it seems a more convoluted proposition.
A
third option: do you notice the plate shaped like a star on the front of the shift drum? It's the stopper plate. That's what you move with your foot. Here is the arrangement of the thing:
As you can "see", the "points" of the star are the place where the "claw" engages to turn the shift drum and engage the gear, isn't it clear? Elliminate one of them and you have one less gear.
About the gear ratios, you can change the sprockets or the rear wheel size avoiding a lot of work...
)