Create a bugtracker as a new application for each of your clients in different sub folders. Create a group for each of your clients. Assign read/write/create permisions for the appropriate group to each instance of the app. When a customer registers add them to the appropriate group so they can only access one bug tracker.
If you want to be more sophisticated you can create a subsite for each of your customers and map it to subdomiains like
client1.yourdomain.com
client2.yourdomain.com
Your customers now have a dedicated URL for bug tracking and anything else you want to put there like ETP or forums.
