How to hide some tasks in All Task List under Activities tab for an Account

Hello All,

I have a requirement where I have to hide all the records for a document type lets say 'A' from some business users. Now I was able to add the document type field in the advanced search query in Task tabs under Activities work center and assigned that query to the business role. So in Activities WC Users with the business roles are not able to see records for document type 'A'. But when you go to any account and check under Activities tab for that account , those records are still visible under All Tasks. How can we make these hidden from everywhere?

Thanks in Adance.