This will take into account subsequent sessions
- without the DEFERRED option, the command will fail

alter system set recyclebin=OFF DEFERRED SCOPE=BOTH;

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PURGE RECYCLEBIN;

To remove all dropped objects from the recyclebin (system wide):
PURGE DBA_RECYCLEBIN;

Drop a table without sending it to the recyclebin. Example:
DROP TABLE tab1 PURGE;