As hard mentioned you can use hlstatx to see when a specific player disconnected. But I think there is no view to see the last people that disconnected. In theory I could provide such a view but what would be the advantage?!
If you want to report them, you usually need a demo cause otherwise admins will tell you that there is no proof if someone rejoined to avoid death or for other reasons. Well there might be well-known rejoiners where you can also see many reconnects in their logs, but those we could easily catch automatically (even though i need to check if we can see the difference between disconnect 'by user' or 'by connection loss'...
So do you really just want to warn them (in that case we could also show a warn message to that player whenever he rejoins) or what are you hoping for?