I have a similar problem, so thanks for that tip about the "Hide" aspect of the invoice. That has cleared a small number of the items that appear in my Unbilled Costs by Job report. However, the majority of my items are not explicitly Billable costs, but rather are General Journal Entries that a past accountant used in order to do some fine-detail revenue recognition[1]. 

I think the problem is simply that when they've made those J/Es, for some reason the accountant has decided to put the relevant customer's name into the "NAME" field in the corresponding line(s) of the J/E. And the reason I think that's the culprit is that I tested it by simply removing the customer's name from the NAME field. When I did that, sure enough the item in question disappeared from the Unbilled Costs by Job report, just as it did following your suggestion to use "Hide" for costs marked as Billable.  To be honest, I don't really see why the J/Es have to have anything in the NAME field in the first place. All they are doing is simply shuffling already-invoiced revenue back and forth in time. That already-invoiced revenue does of course have a NAME attached to it, but that's done back in the actual invoice which we create in the normal fashion. So also doing it in the J/E seems a bit pointless.

One more clarification: in the J/Es in question, most do not have a checkbox in their "BILLABLE?" field on each line. And on the rare occasion where they do have a checkbox, it is always unchecked. So I'm pretty sure that what is making the J/Es in question appear in the Unbilled Costs by Job report is the mere presence of a name in the NAME field. Whether something has been marked as Billable or not appears to be irrelevant.

My question then is, what is the best way to get these J/Es cleared out of the Unbilled Costs by Job report? Should I just go ahead and clear all the NAME fields, as I did to test it? Or will that break things in other places? 

thx

[1] For example, we sometimes have clients who are invoiced weekly. On the occasions where a week straddles a month boundary then the accountant journals so that although there is a single invoice, we end up with two blobs of revenue, one in each month.