The quickest way would be to drop all of the schemas containing version enabled tables. That is assuming none of the other objects need to be kept.
It is recommended to execute dbms_wm.RecoverFromDroppedUser after doing so to clean up/fix any metadata. This only needs to be executed once after all of the required schemas are dropped, not after each drop user command.
If there are tables or other objects that need to be kept, then you would need to use dbms_wm.DisableVersioning.