Delgado had a really hard time ignoring Zazu, while he's a very dog savvy cat he'll run rather than fight and Delgado loves to chase. I found there were two things that really helped Delgado:
1) I made myself more interesting than the cat! The cat may run but I have treats, an exciting voice, toys, etc. Why run after the cat when you can come to mom for fun and get rewarded for it?
Keep a tug or ball on you at all times and when you see him bugging Harley call Varick to you and reward him with play and attention when he comes. He'll soon learn that it's more fun with you then with Harley when Harley isn't interested in playing
2) I kept him leashed to me when he was younger, whenever I noticed him focus on the cat I would verbally correct using a "leave it" and distract him. The leash preventing him from running after the cat so he learned that trying to run gets you nowhere and that leaving the object alone is more rewarding.
I hope that helps, consistency was the key I found above all
Jasmine - Female Miniature Poodle - born Aug 15, 2010
Loker Delgado Von Stalworth - Male GSD - born Jan 26, 2012
Koda & Zazu - 5 year old male cats
Alex - Male Cocker Spaniel (rescue) - RIP Cuddlebug 2007-2010