Is it possible to remove DSNBS that have been used at one point but are currently in the free chain?
You can't remove DSNBs that have been used with the TMSXTEND utility program.
If you need to do this, you will have to use TMSSPLIT to export all of the tape volumes and associated DSNBs, format a new TMC with the reduced amount of DSNBs, and then run TMSMERGE to rebuild the entries.