I don't think there is a way to eject only full tapes natively from within NetBackup. You would have to do some scripting to achieve this.
NetBackup 6.5 and higher does provide a couple of mechanisms to help reduce the number of media that get sent offsite. I was originally sending around 8 tapes a day for data and now I send around 3.
Rather that retyping the options, this thread has a couple of suggestions
Use the settings for MPF and create server groups for your media (if you have multiple media servers). For MPF, you can start off with a low setting and then throttle up as you run out of time for duplications.
Another thing to note, as a start, is that the following can't share the same media.
1. Images with different retentions (by default but can be changed)
2. Media duplicated by different media servers (can be fixed by setting up Server Groups)
3. If you are vaulting to different volume pools, these will stay separate (of course)
Take a look at the following threads for some extra information that should cover what you need.
https://www-secure.symantec.com/connect/forums/vault-duplication-many-differente-retention-backup
https://www-secure.symantec.com/connect/forums/how-do-you-estimate-how-many-tapes-will-be-needed