Hi Rafal ,
You can expire tapes with "vmquery" command. Steps are like this ;

Step I: Looking media properties ;
vmquery -m [Media_ID]

After this query your are going to see some info s . Note the Volume Pool 
number (it is number).

Step 2 : Expiring tape (vmquery command);

vmquery -deassignbyid -m [Media_ID] [Volume Pool Number that you have noted] 0

That's all , your tape will expire.!!