In the K2 Management section of K2 Appit for SharePoint you are able to access the SmartObject Security section:

In this section you are able to view the administrative rights for SmartObjects:

Here you can add users to this list but it should be noted that when you add a user to this list only those listed can create SmartObjects. This means that any other users who try to create SmartObjects will be unable to unless this list has no explicit users in it.