You should be able to 'offline' the tablespace in question (Alter tablespace undoTbs1 offline). This will allow any open transactions to complete at which point the tablespace will be taken offline (this may take some time depending on what type of transactions you have going). You should then be able to drop it. I had this problem a while back with rollback segments on an older version database and it took me a week to get all the old rollback offline so they could be dropped...just a lot of long running transactions and a very active database.
Check your alert log file and if you getting "Undo Tablespace X moved to Pending Switch-Out state." error message than you need to find the pending transactions which blocking you to drop UNDO tablespace. Use query below to find the pending offline transaction.
You can either kill the transaction or call the user to commit his/her transaction. After this you should be able to drop UNDO tablespace.