On the last cassette I took apart, the "rivets" were actually bolts with tiny hexagonal heads. Try turning them out with a pair of pliers. You'll want extra spacers to replace the space left from removing gears.
It's not a great idea to take off the derailleurs and shorten the chain. Most modern cassettes are ramped, making for smoother shifting, but if you had a shortened chain on a full cassette, it's not all that hard to accidentally catch one of those ramps and "shift." I had a friend who broke his axle in a situation just like that.