Deactivating the user (by going to hipchat.com/admin/users) should behave just like deletion (and allow the user to be recreated via other means, like an import). If that isn't working, however, I'd recommend contacting the support team at email@example.com and they should be able to more fully remove the user so that the import works properly.
...have mention name FirstnameLastname. Fe.: @MaartenCautreels This certainly helps to know who is being mentioned in a conversation. Imagine your colleague sends the following message...
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