Are they senior, junior or equal to you?
If they are doing this enough of the time to mean they're not pulling their weight it needs dealing with. If they're junior you're obliged to talk to them about it. If they're equal you could couch it in terms of "you might want to nip this in the bud before you get caught". If they're senior, you need to report it to someone they're equal to, or their manager. In all cases, you need to ensure others have noticed it too and support you.
Ahem - I'd, er, better get off here and do some work.
As should you - slacker!