Security roles and Activities

Hello everyone,

since my last problems with security roles and subcompanies, I've got a new question.

Situation:

Parent Company 1 - User Mike

Child Company 2 - User Tom

Tom has created an account "Company Zulu" in Company 2.

Mike creates an activity in Company Zulu.

So here is my question, how can I hide the activity for User Tom and his colleagues in Company 2?

In the Formular I can do this with filtered views and a JavaScript function which hides the social pane activities.

But when I click on my work -> activities, Tom could instantly see the activity from Mike and that is the thing I don't want to...

I've also tried it with changing the owner of the activity from user Mike to Team Company 1, but it doesn't work either.

After a few more tries it seems to me that Tom sees every activity that Mike creates in his Company Zulu.

Thanks for help,

regards,

CRM_Believer