The solution for Alfresco 5.2 version is to add the configuration to the alfresco-global.properties:
activities.post.ignored-types=org.alfresco.documentlibrary.file-previewed,org.alfresco.documentlibrary.file-downloaded
More informations:
https://community.alfresco.com/thread/228224-how-to-hide-document-previewed-in-site-activities-dashlet-52e52d