In our organization, we wanted that some of our users should not be allowed to close an opportunity. In the user roles we didn’t find this specific privilege. Hence, we removed the “AppendTo” privilege from the opportunity entity for the role of those users who are not allowed to close opportunities.
Thereafter, whenever that user tries to close an opportunity for which he does not have permission, he would get the following message of “Insufficient permissions”
Note: Having done the above privilege changes, this particular user would not be able to associate any type of activities with opportunities.