I am not a big fan of 'redirection'...I really do not feel it is clear enough to a dog. The matter must be black and white. The cat MUST be off limits. The dog must learn "leave it" and if it means some physical correction, then so be it. Start with the scruffing and aggressive correction in your tone of voice. Separate the pup from the cat. The pup must learn - for the safety of the cat - that the cat is NOT a chew toy...just look at what pups can DO to a chew toy!
Some dogs will never learn to leave the cats alone...I sold a very very very nice male because he was obsessed with cats and it got to where he just drove everyone nuts in the house pacing and whining at the entrances to the rooms where the cats holed up when he was inside. I don't like dogs living 24/7 in crates and kennels and the cats are just as important as the dogs...I did not spend enough time when he was little establishing the parameters of the cats being off limits...
Csabre Sch1, Hexe Sch2, Bengal, Komet, Kyra, Kira & Kougar v Wolfstraum ~ Ziberia BH ~ Danger Sch1 RH SAR ~ ATB~ Kougar Sch3, Kyra Sch3, Cito Sch2, Alice Sch1, Kelsey CD, Ret ~ Basha Sch3, Fenja Sch3