Validation rule for Plant-Company code posting

Hello Readers,
I have a scenario in which I have to restrict plant posting when i was posting company codes. Let me give the details.
Lets us say, I have to post a journal entry for a company code A and plant 1 only. We should not allow users to post journal entries other than Plant 1 when you are posting to company code A.
I am thinking of writing a validation saying that only plant1 is allowed to post when posting company code 0061.

I went through configuration transaction code OX18. But even if assign company code A to plant 1, system is still allowing me to post for other plants when I am trying to post in company code A.

Please share your thoughts on this.....

Thanks
Pavan