When we queue a case to a standard agent for processing, it creates an entry for the table corresponding to the class System-Queue-DefaultEntry.
If you need to de-queue an entry, it is same as removing (Obj-Delete) the particular instance of System-Queue-DefaultEntry(Please see the xml of any System-Queue-DefaultEntry instance to know the property in which it has the pzInsKey of actual case).
This is applicable only if the standard agent has not executed the agent activity on the case.
If the entry is already processed or being processed, we cannot de-queue as the entry will be removed immediately after processing.
We can even use the pega API to add some methods that can be used for de-queing which are present in PRQueueManager interface: