1. Using the Operations Navigator
Click on "File Name" column to sort by name (and date). Delete any old/duplicate entries, job logs, etc.
An alternative is to use the selection capabilities for deleting more than one spool file at a time.
2. Using WRKSPLF
For example enter a value in USRDTA if all desired spool entries (and no others) have the same USRDTA. USRDTA is a column in WRKSPLF that can be viewed.