A global result is not tied to one or more Interceptors or Actions: it is global to everyone, so it will be rendered when returned, no matter who is returning it.
You need to add a global (keeping it consistent with your other login result, that is global) result of type json, like this:
<global-results>
<result name="login">/admin/login.jsp</result>
<result name="loginAjax" type="json" />
</global-results>
Then, you can send a particular statusCode, or errorCode, like described here.
If you need to understand what kind of result you should return from within the Interceptor, you should proceed like described in this other answer, by checking the type of the result that it would be returned in case of success.
P.S: Oh, and consider hiring a Java dev to do Java stuff properly :)