How to delete all custom objects for a specific class?

Records can be deleted only by their owner. You can delete all objects for a specific class in three steps:

First you need to retrieve all objects by QBCustomObjects class:
objectsWithClassName:extendedRequest:delegate:

Next create array, going through all items in a loop and collect their item IDs.

At last perform deleting by QBCustomObjects class (and pass array of IDs):
deleteObjectsWithIDs:className:delegate:

Detailed information about this method you can check here http://sdk.quickblox.com/ios/Classes/QBCustomObjects.html