To stop receiving traffic from a source for which you haven't found a bid level that works for you, we suggest inactivating it from the Traffic Sources view.
To access the Traffic Sources view in dashboard you can click on Traffic Sources from within a campaign.
You can then inactive specific sources by clicking on their status. You can also inactivate them in batch by ticking the checkboxes of multiple campaigns and clicking on Batch Edit next to the date picker. Remember to select Status as the field you want to edit.
Alternatively you can edit the campaign and exclude those sources from the targeting settings of the campaign. The end result will be the same but using Traffic Sources gives you a better visibility in our opinion so we recommend you use that instead.