In ChastiKey 2.5.2+ you can follow and block users. Any users that are on your blocked list will also be blocked from loading your locks.
Your list of users you've blocked can be found from the 'Friends' option on the app menu. This will take you to a screen showing your followers, who you're following, and who you've blocked.
At the moment you can only block users you've interacted with, and only by viewing their profile and selecting block user from the top right menu.
Blocked users won't know that they are blocked by you as such, but they will see a message on your profile saying that your profile is private.