You can't decide for yourself what the semantics of someone else's address do or do not mean.
It might make them bad netizens and you may not like it. But the spec doesn't compel any behavior. It's just a way to communicate technical ideas and ideals.
You're under no obligation to accept names that are hard (as in painful) for you (except if you're the police, I suppose. Ironically)
You can't break user expectations and mental models by pointing to the spec as justification. The spec exists to serve users, not the other way around.
In the absence of being able to count on specs, I guess the user should expect a race?
Most of them can't specifically point to the problem, their impression is just that your services don't work properly. They're right.
Assuming they don't just fail to register and move on ...