You are not able to delete the records permanently and the records will be stored into the Recycle bin till 15 days from the deletion date:
Number of records stored in recycle bin = 5000* Number of Licenses your organisation has pursed.
If you want that all the records are deleted permanently from salesforce recycle bin then we can use this method:
EmptyRecycleBinResult[ ] = DataBase.emptyRecyclebin(List of Ids)
Account Acc = new Account(Id='xxxxxxxcxxxd000000s4'); Database.emptyRecycleBin(Acc);