First, it's a good idea to understand the difference between a Time & Sales entry, a bill batch, and an invoice. A Time & Sales entry is the first step to getting your hours into the system. Then, when you choose to do it, Time & Sales entries get Bill Batched. Bill Batches can then be turned into an invoice. You can delete an invoice without affecting the bill batch, and you can undo a bill batch (or remove a specific client from a bill batch) without affecting the Time & Sales entries.

You can delete an invoice from two places, both of which can be found on the Invoicing Screen, under the Accounting Center. In the Bill Batches tab, simply click the delete link in the Command column underneath the invoice number (you'll need to click the little plus sign next to the bill batch that contains the invoice in order to see it). In the Invoices tab, simply click the delete link under the Command Column.