You really can't prevent the shared password issue unless you billed on a per login basis. The bottom line is that you end up accepting that you grant access to a user you give up controlling the sharing. You can only prevent multiple login's which is pretty good. It really does not matter if two people use the site 12 hours per day or one 24 hours per day.
You can track IP's to look for suspected sharing i.e. use ARIN / RIPE look-ups and see if the same user log's in from multiple subnets. I would think that this would be quite easy with PHP and a MySQL type log.
Do your best to limit abuse and move on.
Dan T
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