First which you need to do is creating an marker aspect.Apply it to the folder on which you want to disable the download action.There are multiple ways for adding an aspect.
For disabling the download action on the children/files which are inside the folder we have 2 approach.
First one is,
Creating new custom evaluator in which you need to check whether the parent folder has the marker aspect or not.If it has you can disable the download action by returning fals evalue in custome java evaluator.
Below link will help you in creating an custom java evaluator.