If you have received a supplier invoice that is not to be assigned to a job, click the ⋮ options button next to the invoice on the Supplier Document Processing page and select the option to Send to your accounting package Only:

This will send the invoice across to your accounting package and remove it from the supplier document's processing screen, but not assign it to a job.

If you need to find that invoice again later, it will still be present in the Supplier Documents Report.