Is there a way to delete related but undeleted files of removed applications?
I use a nice little free app called App Cleaner for the same reason/purpose. (I am not affiliated with AppCleaner. Note: I was asked by Dan to insert a disclaimer when suggesting a product I am not affiliated with.)

It has been very effective in finding and deleting all associated files and gives you the option to view and deselect certain files, if needed, prior to deletion.

The App Cleaner is available on the App Store.

From the developer:


  Main Advantage: App Cleaner deletes Remains (or leftovers) - files of
  previously removed apps. Thus, you uninstall applications completely.
  
  Main Features: 
  
  
  Scan the system and preview installed applications
  Remove all types of service files
  Remove remains of previously uninstalled apps  
  Reset application to a first-launch state
  Easy-to-use shortcuts
  Any file or app quick preview
  Specific app-associated file formats preview
  In-app help assistance
  Show unused applications
  Skip list option
  
  
  How to Uninstall an application using App Cleaner:
  
  
  Launch App Cleaner.
  Scan all applications for service files.
  Choose unneeded application (left view). In-app purchase required.
  Select all its service files on right view.
  Click Remove.
  If some apps were not removed, due to access rights, switch to Finder and move them manually to Trash.