You'll probably have to start by removing the whole throw from the drum. The old levers were made of brass, and the part it threads into is made of steel. This is good since there is seldom rust holding them together. If you're lucky you may be able to use longnose pliers to twist the piece out if there is stub from the lever poking past the steel part. This worked for me the one time I had to do this.

If not it is much harder - you'll have to use a very fine drill to drill a hole in the remainder of the lever and then get the piece out with a very fine stud remover.