One solution is to expose unique IDs and allow people to hang nicks on those. Using email addresses or phone numbers goes most of the way toward this (it's not all the way, since people could get an email address or phone number that someone else used to have, at some ISPs or phone companies). However, there's a larger, related problem, which is the problem of identity: there is no foolproof way of identifying individuals, and the more you investigate, the more clear it becomes that existing attempts fail miserably for edge cases, and criminal / fraudulent activity is probably the longest edge.