Is there a way to prevent a user to buy other products (selected ones by admin) when a user has bought a product X?
 
E.g. There are products A, B, C and D. When user has bought D, he should not ba able to buy A, B and C anymore.

OK, I've played with "Users Types" by adding a new user "Not logged in" to a category. But unfortunately this does not work as it will show categories when a user type is logged in and not show them when not.
 
I need the other way around where categories are shown to all and NOT shown when a customer is logged in.
 
Any idea, or am I thinking wrongly and the above is also possible?
 
Help appreciated.
 
DickS