How to purge an already dropped table?
PURGE TABLE test;


or

PURGE RECYCLEBIN;


From docs:


  The following statement removes the table test from the recycle bin.
  If more than one version of test resides in the recycle bin, Oracle
  Database removes the version that has been there the longest:

PURGE TABLE test;

  
  To determine system-generated name of the table you want removed from
  your recycle bin, issue a SELECT statement on your recycle bin. Using
  that object name, you can remove the table by issuing a statement
  similar to the following statement. (The system-generated name will
  differ from the one shown in the example.)

PURGE TABLE RB$$33750$TABLE$0; 

  
  Remove the Contents of Your Recycle Bin: 
  
  Example  To remove the entire contents of your recycle bin, issue
  the following statement:

PURGE RECYCLEBIN;



http://docs.oracle.com/cd/B19306_01/server.102/b14200/statements_9018.htm