There is always one definite method to tell if someone is spoofing. Your hosting a map, and a person named "&&&" joins. Your pretty sure its a spoof, so you type this:
/Whereis &&&
&&& Is Not Logged On.
Although it doesnt happen all that frequently, if you know the real name of the spoofer (ie, maybe he joined last game), you just need to type /Whereis [RealName]. Wham bam, thank you ma'am, he's caught.
Of course, like you guys have said, if you dont know his real name, its pointless.
Zera: by custom name, i mean you could have stuff such as " ( {(*&^$#@ and all that good stuff in your name.
Thats called an "Illy." Its a name with any illegal character, but still exists in the B.net registry.. or whatever. People really dont care if you join a game with one, as most people know about the /Whereis thing.