A lot of people put their backups on S3, with a script running on the server. Even if you limit the rights with IAM to only put files, the attacker can overwrite existing files on S3. The only way I thought to prevent that is to give only write access with no listing access, and append a random number to the file name. But, who does that? I'm sure 90%+ of the servers backing up on S3 are not safe for this scenario.
If you turn on file versioning in S3, then you'll be able to get to the data that was "overwritten". I don't think there's a way for someone with only PUT access to work around this.
The reason I thought of DVDs is that they're not sensitive to electromagnetic fields as disks and tapes. (You never know: http://www.telegraph.co.uk/science/space/9097587/Solar-flare... )