Install the TB recaptcha module.
Also you can lock your folder with a password - they have to bruteforce this password first, if it's strong enought this will cause some server load but not as much as having your admin open to the public.
Then you have google's recaptcha, use 'strange' admin email (NOT admin@, sales@, office@ john@, etc) and strong password.
I doubt anybody will be entusiastic enough to go through this.