This should be happening automatically (tapes with 0 vimages should expire and 
return to Scratch automatically).  Try vmquery -deassignempty and see what 
happens.  If you get an error, there is likely a problem with this process 
which should happen 2x daily by the system automatically.  You should then open 
a support case.  They'll ask for some logs and narrow down specifically what 
the problem is (i.e. images/tapes in the database associated to a decomissioned 
media server that is hanging up the process).