@Override
    protected AuthenticationHandlerExecutionResult authenticateUsernamePasswordInternal(
        final UsernamePasswordCredential credential, final String originalPassword) throws Throwable {
        val username = credential.getUsername();
        if (this.users.contains(username)) {
            throw new FailedLoginException();
        }
        val principal = principalFactory.createPrincipal(username);
        return createHandlerResult(credential, principal, new ArrayList<>());
    }
The collection of users to reject.
support/cas-server-support-generic/src/main/java/org/apereo/cas/adaptors/generic/RejectUsersAuthenticationHandler.java