The sessions that are showing up in your "Unbilled Sessions" list are sessions that are set with the Billing Type "Insurance Pays Me". If you have already billed insurance for these sessions, you need let the system know so that they're removed from this list.
The way you can "let the system know" that these sessions have already been billed for, is by adding them to a claim form. You don't need to submit this new claim you create for these sessions, all you need to do is create it and Save it. This way, our system will understand that these sessions have already been billed and will be removed from the Unbilled Sessions list.
The best way to create these claims is to go to each one of these client's Billing page and select the date range that will include those sessions. Then, create the claim from there and Save it. Once you do this, you will see that the sessions in the Unbilled Sessions list will be gone and that will prevent you from accidentally adding these, already billed sessions, to any claims you're going to be filing.