Need more information..
I’m guessing from your post that you have two partitions on the same physical system. And you have BRMS running where?
When you try to delete you should be using command WRKMEDBRM (or GO BRMMED and use option #2). Are you getting some error when you try to delete?
If you “display” using option #5 in WRKMEDBRM what do you see for the “system” on the volumes you are trying to delete – and what about the other volumes?
If you have BRMS handling both partitions and you really want to “allocate” certain volumes you might be able to specify the system for the tape. I think you also could (1) create a new media class of the same density for yout tapes (Go BRMMED, option #1) and (2) assign volumes to that class. (3) copy the media policy you use for the partition and change the copy to use the new media class. (4) change the backup control group you use for that partition to use the new media policy.
Last question, if you are using a media library and barcoded tapes (you didn’t say what you had), do you care about what volumes are being used for what? BRMS will use the scratch tapes as needed and should report to you what was used.