At Kevlarmans recommendation, the slot numbers are already banned names on my servers, that way noone with a number for a name can join. Slot problem solved.
I suspect that the same could be done for anyone with a strange character for a name, such as ~. If you have a problem with kicking someone with a strange name, I suggest you tell an admin and/or operator, they can get the character used from the logs and create a preemptive ban for the name so that it can no longer be used. Hopefully the ~ name ban will work, I'll have to test it later.
As far as duplicating the name of a player on the server, perhaps it is time to kindly ask one of the many coders to create a patch that disallows it. Maybe it could be a part of or an addition to Foobar's NewbieNaming patch, so that even Unnamed Players do not have identical names.
If an admin is on they will be able to see slot numbers and the like, your complaints only really affect user vote kicks as far as I can tell. As the person who should be kicked has no idea who the admins are, they really have no idea what they can get away with. Maybe lots, maybe nothing. If they know that there is no admin, they know that they can get away with more and for a longer period of time. Basically, you won't be able to tell if you are on a server with admins or without.
If they are just morons, well, you can't really help that :-P