This only applies to Cost Plus jobs and Time and Material service tickets. If your job is Fixed Price then the time will automatically not be billable.
If you have time entries against a job that you want included in the cost, but not billed to the client, you can turn off the "billable" feature in the time entry when reviewing time. First go to the Review Time screen and find the entries you'd like to edit:
Then once you uncheck the box in the Billable column, save the time entry:
This will then leave the cost associated with the project, but not bill the customer for the revenue.