If you want to delete some of your leads, but not all of them, you can do this as an administrator.
First, you will need to go to the "Campaigns Tab"
From there, click on the "Remove Contacts" icon that is within the campaign that you want to remove leads from.
This will open a new window.
From within the new window, place check marks next to the dispositions that you want to have removed.
This means that any lead that has the disposition that you check, will be removed from the dialer.
Once you have selected the dispositions that you want to have removed, click on "Dispositioned As" to remove them.
You will still be able to disposition calls with those dispositions, you will just have removed any leads that had those dispositions.
This can be used to remove leads that have bad or disconnected numbers.