What to do with banned links — in link shortening service?

it is better to never re-use links — they can still remain on public domain, and someone in the link can transmit the username/password (well, with links to google docs access the link)

Better user came to this link to say that it is banned, than to force him to wander the site and look for something with which he moved...

Better not sure you have a large supply of free links.

No need to release the references. In this sense zero. Disconnect the 5% of the registered links and the inconvenience will create a users mass.
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If the hash links are used Latin letters in both registers and digits, six digit hash will be ~ 56 billion, five — 0,9 billion. You very soon all of them will take.