If you are using 7.1 you need to use virtual path provider to programmatically work with files since contentrepository will only work with files from 7.5+. Try first to change the regex that validates files and see if that lets you delete it with file manager. Else do a scheduled job or similar that use virtual path provider and delete it programmatically. 

Check out 

http://world.episerver.com/documentation/Items/Developers-Guide/Episerver-CMS/7/File-System/How-To/Accessing-Files-and-Folders/

For how to work with these. It should be something similar to

    UnifiedFile file =
        System.Web.Hosting.HostingEnvironment.VirtualPathProvider.GetFile(path)
        as UnifiedFile;
    file.Delete();

...but I don't have a 7.1 environement up right now to check.

If you are using native provide type you can just delete the file.