1. Create an Event Lists report
  2. Mark to create an output constituent query
  3. On the Filters tab, choose Selected Registrations and then 'Not Registered'
  4. Set all other preferred parameters
  5. Run the report
Note:  By selecting the checkbox to create an output query,  a query will be created for later use

OR

Create a Participant query with the following criteria: 
Event ID equals name of Event, Invitation equals Invited and Registration equals Not Registered