Save Set control

You can't. Data on tape stays there even after deleting it from mdb.

You can only reuse the cartridge again when the tape becomes relabelled. If the proportion of the tape this uses is large and you have a desperate need for space you can stage the remaining savesets to another volume and then relabelled. Use the mminfo command to list details of the savesets and get the ssid/cloneid then use the nsrstage command to stage the data, then relabel the tape as normal afterwards (first verify there are no valid savesets).
Also be careful in thinking a large saveset is on the tape as it may span several tapes!