You may not have any outstanding transactions, but you may still have to purge the Final Billed transactions. From the Maintenance menu is a function 'Purge T&D Ledger', where you select the specific customer and matter(s), and also click the Final Billed checkbox.

Once these have been purged, you may also need to ensure that no transactions exists for that client on the A/R side - have they all been paid or otherwise dealt with? After these steps, you should be able to delete the matters and after that, the client.