Replace a file in use by another user

As server administrator I update a virusdefinition file automatic several time each day from an external server – replacing the old file on the server (a combination of batch-file / FTP-command).
However – sometimes the existing file is in use by an other user – who has started his/hers own updating and then left his computer (and task) in the middle of the process avaiting user input.
Therefore the file is in use – and my automatic task is aborted.
Is there a way around this ?
Is there a way to automaticly “free” the file from its lock – from inside my batch-file running the download/updating.
Regards Geir