Once you have created an iCloud account and chosen an @icloud.com address to go with it you can't change or delete the address . However you can add up to three 'email aliases' - these are additional addresses (not accounts) which deliver into the same inbox as the main account. (In fact it's a good idea to give out alias addresses, rather than the main address, because if they attract spam you can easily change them.)
You should be aware before you start that once you've created an alias you cannot turn that address into a full iCloud account or move it to another account.
More information on aliases here: http://help.apple.com/icloud/#mm6b1a490a
As to the spam coming to the main address - you can't delete the address but you could set up a Rule to move all incoming messages to that address to the Trash, from where they will be deleted after 7 days. Click the cogwheel icon at bottom left in the Mail page and choose 'Rules'.