WorldEdit is an open source plugin that you can have a look at, it does more than just a radius of x amount.
If you've used WorldEdit before, then it really isn't complex if you know what you're looking for. You want a radius of x amount. Have a look at a command like /butcher that allows you to set a radius too. To find your way around commands, look for the com.sk89q.worldedit.commands package. You don't learn2Java by lying around.