We never recommend deleting records in your system. Instead we recommend that you simply inactivate them. That way you can retrain the records for future use, but they will only show up in your searches and on your reports if you specifically choose to include inactive clients.
In order to inactivate a client, you first need to pull up their record by searching for them. Once you are in the client record click on General Information. Then uncheck the "Is Active" check box. Click Save.