You appear to be writing a script that finds records. There is indeed an option where you can enter "Completed" and specify the omit option to make the entire request an "omit" request as well as the ability to create multiple requests so that you can specify other criteria if needed and then add an "omit" request to filter out all records where "completed" is the text in the disposition field.
But in a script, I prefer this approach as it makes it easier to see what search criteria was used in the script when you review your script at a later date:
Enter Find Mode---->Clear the pause check box
Set Field [YourField::Disposition ; "Completed"]
Set Error Capture [on]
Perform Find 
When your script is in Find mode, Omit Record produces the same effect as clicking the "omit" button in the tool bar.