Unfortunately, the Query Editor does not have a "not" operator: I'm taking note of that request though and adding it to our backlog.
As a possible solution, the recently released Wwise 2021.1 offers WAQL (Wwise Authoring Query Language) that you can use in the search field of the Authoring. For your case, a WAQL query that translates your query would be:
$ where ! name = /^step/
For more information about WAQL, see https://www.audiokinetic.com/library/edge/?source=SDK&id=waql_reference.html.