I am also struggling with the same issue, where they post invalid codec movie, usually size of 700MB, but if you download and rar, it will be merely 5MB, these rar files only contain some random text to achieve 700MB size (in a affected movie file).
What i have done?
1. Restricted Submission from normal members.
2. If you want to submit anything, you need to apply by Contact us system.
3. Moderator scan newly upgraded uploads,
Good Solution:-
The possible good solution is to create an automate script which download the rar files automatically, extract and re-rar and if size of files differ in more than 30%, then definitely it is virus!