If they are in danger of their nick being reaped, the Node Reaper should send an email giving them warning and a certain amount of time.

If the email address is invalid, or there is no response, logins or posts, then it seems reasonable to return the nick to the pool.

I am strongly against cybersquatting in principle, and this seems to be a similar issue.

For the future, I suggest a paragraph on the registration page which makes it clear that unused Nicks will be deleted after X time unused.
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Brovnik