Multiple file uploaders

[eluser]Aidy[/eluser]
Sorry if this is a stupid question!

I'm looking into using uploadify at the moment and have been looking at this article

http://www.reloadedpc.com/code-igniter/j...deigniter/

The script places the copies the files uploaded over to a directory. Unlike files associated with the $_FILES array they are permanent. My question is what's to stop someone form uploading lots of files to your server at once filling the hard drive right up?