You can customize the message the user will be presented within the messages bundle of a theme (Themes > Your Theme > Edit > Messages). In this case, you will want to modify the following message key.
[UserLockedException]=Your account has been locked. Please contact your system administrator.You can modify this message to your requirements
[UserLockedException]=FooBarBazYour modification will be displayed to the end user.
found variables for other messages displayed to the user as part of the Login Flow, but not for the Account Lockout Message.
I find exporting the entire message bundle first (by "selecting all" <CTRL + A> from the admin UI) and then pasting the result in a code editor to find a message key is sometimes easier. We do have some adjacent documentation here.
For reference only, our Theme API is also available here