Anjelina,
Remember that this is the Flex API forum and not the Flex Viewer forum:
http://forums.arcgis.com/forums/111-ArcGIS-Viewer-for-Flex
where this question belongs.
You can control what widgets are available to users based on different config.xml files that have the edit widget removed. Many people have a custom ASP.net front end to their viewer that handles login and then based on login they are directed to the appropriate config.xml when the viewer is launched. As far as something out of the box "widget security" there is NONE.
Anjelina,
Sorry I do not help with Visual Studio or .Net questions, which is what the answer to your question would involve.