The route that allows you to delete backups without disabling Image Guardian and in the safest fashion, i.e. without risk of creating orphaned backups, is within Reflect.  Go to the Restore tab and you should see your backups there.  If not, click "Folders to search" and add the folder containing your backups.  Then select any backup and click Other Options > Delete file.  The dialog that opens will show you all other backups in the same set as the one you just selected and allow you to delete whatever you want.  This is a bit better than Windows because you'll notice that if you select a parent backup for deletion, all child backups will be automatically selected.  This gives you a visual indicator of the ramifications of deleting a particular backup and again makes sure you don't leave any "orphaned" backups, i.e. backup files that are now useless because a parent backup has been deleted but are still sitting there taking up space.

But if you don't want to use that interface, temporarily disabling Image Guardian is the way to go because you can't execute a retention policy without running a backup.  I suppose you COULD set a retention policy, enable "Run purge before backup", then run the backup and cancel it after the retention rules run but before the new backup succeeds, but that's unnecessary hassle.