Hi ishaan
Permission to upload files is determined by the user roles. As an administrator you can change their role. Users can upload files as Authors upwards so to stop them uploading you should make them a contributor or a subscriber.
You can read more about it here
http://codex.ww.wp.xz.cn/Roles_and_Capabilities
If the default roles don’t give you what you need you can try a role editor plugin like this one http://ww.wp.xz.cn/extend/plugins/user-role-editor/.
Hope this helps.
thank you sir for your help but is there some kind of way by which users can connect via facebook and upload their files??
Simple Facebook Connect (http://ww.wp.xz.cn/extend/plugins/simple-facebook-connect/) by Otto will integrate Facebook photo albums and the media uploader.
