Here are a few things you can do
- Kill session from GUI
Go to toad and log on as system administrator. You could also do this sql developer.
Now you would have an overview of the sessions that might involve in locking the tables you would like to drop. So you could simply identify the sessions and kill the corresponding sessions where table locks are withheld.
Check v$session and kill session from command.
However sometimes, even we kill the existing sessions, the error would still not go away. I run my drop table script in toad that you could get the details of which table is being locked and the type of the lock. And then go to locks tab to identify the Sid number. You would see a similar window as below.