I completely get why the system can't refuse names that are similar, only names that are identical.
What would be good is if, as a new member types in their chosen nickname on registration, a list of users with similar nicknames was generated. In this way they would at least see that there are others already registered with near identical names and they could then perhaps think about choosing something else.
I've been on forums where there is a search facility for members, and the entire member list is generated. I can't think of a single reason why that would be a bad idea, though I'm willing to be persuaded why it could be.