Networking in General
This question is not really about Linux Networking, it is general networking. I currently administer a network with about 32 users. Recently, one user setup a router incorrectly and has started issuing out IPs. This rogue dhcp server needs to be taken down. I have identified it; however, is there an ethical way to approach other than isolating the connection and severing it. Also, is there anything I can do to prevent this from happening in the future. Educating the users would be appropriate, but I am in no position to force knowledge on them. In my experience the only way to deal with this is to disconnect the user and/or educate them of the problem and help them fix it.
It really depends whos network this is. If you're the primary sysadmin for the network, then just cut them off and force them to reconfigure before reconnecting.
If someone's using a network for which you're responsible, then your responsibilities lie first and foremost with the users of the network; a rogue router or server hardly helps you do this. You have to make it clear that the network integrity is your responisbility, and you dont take it lightly. If they dont know what they are doing, they should either ask for help, or go on the appropriate courses. Note, that in this scenario, you are in a position to force knowledge on them - if they want to play with this stuff on your network, then they must understand what they're doing or you wont let them connect.
There's nothing to stop you being helpful and nice to this user, of course, but you gotta make it clear that their tampering will make the network unreliable for all the other users.
As a rule, when things start getting that big, and users start becoming that obtrusive, it's time for a big change. The root password will only be known by you. That's really the only way to ensure end users aren't doing administrative tasks. It will increase your workload, and your calls, but you'll need to measure that against things like this happening. It's possible, in the right environment, that merely the threat of that is enough to make everyone behave themselves, and you won't need to go that far.
Really, that's what the root password's for! :p