Nothing will stop someone who is determined enough to get in.
Auto blocking IPs after X attempts will block most automated scripts and dummies on /wp-admin/. Wont stop someone with
a huge botnet and w WP cracking script.
Renaming /wp-admin/ to something else will stop all autocracking/bruteforce scripts
Deleting user Admin and making the Admin username something else besides Admin
If you want to get more hardcore than that, you can restrict the wp-admin access to your IP only and that should stop anything.
Then you have weak plugins that could always get hacked so thats another way in.
I've slowly been migrating away from WP for anything that needs to be 'secure' and is going to be getting a lot of traffic/be important. GL.