How hard is it?
 

I have a 21 speed mountain bike. I'm thinking of removing the large chainring in the front since I never use it and replacing it with a chain guard ring (if that is what you call it - it's just a plastic disk - something like this: http://www.amazon.com/gp/product/B00...pf_rd_i=507846 ).

I have no experience removing chainrings though. Is it realistic to do this myself? Could something like the product above simply replace the spot where the chainring is now? I realize that I will also have to set the limiter on the derailleur too - I'll have to figure that one out later.

I haven't installed the guard you are looking at, but removing and replacing a chain ring isn't hard. I've seen 3 types:

1) Riveted rings: these can't be removed.

2) Rings fitted with allen keys on front and back: for this, you need two allen wrenches that fit the hexagonal holes on the front and back. The chain ring bolts screw into each other. Hold one wrench steady and turn the other one counterclockwise to remove.

3) Rings fitted with allen key on outside and slot on inside: for this type, you need a large flat screwdriver or a chain ring bolt spanner (Park Tool CNW-2) to hold the slotted bolt steady while you turn the allen wrench counterclockwise to remove.

Riveted rings can be removed, you just need a drill. Then either tap or more likely low profile hardware to reattach anything else there.

The item you linked to is a chainring with built-in chain guards and is for one-piece cranks so isn't what you are looking for. The chain guards I've had on bikes bolt over top of the big ring so you wouldn't need to remove it. If you do remove the big ring, you'll want to adjust your FD's high limit so you can't accidentally shift to the guard.

Why bother?

i'm with this comment.

Mainly because my pants keep getting snagged on the chainring and I'm not a big fan of rolling up my pant leg every time I ride my bike.

Wear shorts or lederhosen.

you'd have to disable the shifter from being able to shift there, otherwise if you mistakenly shift up your chain will just fall off and you could get into a world of hurt.

you are better off just getting a chain guard.

You can adjust the derailer and grind the teeth off the outer gear to make it a bash guard for free if you want.