A more effective and better way to teach a boxer not to chase cats. It is very possible to teach this without ruining your dog and their playfullness, but it takes alot of work.
The first step is management- do not allow your cat to be chased by the puppy at all. Have puppy wear a long, thin leash so you can stop any chasing immediately and redirect the puppy to an appropriate activity. Make sure they are not left unsupervised together, the more times the puppy "practices" chasing, the harder it will be to break. In addition, reward for any calm behavior around the cat with click/treat.
While doing the management, work on teaching a very strong leave it. Start with a toy or treat on the floor- stand with puppy on leash so they are just out of reach of the "prize". Once puppy looks (even very slightly) away, click and treat. Repeat this until you feel like they are looking away practically immediately, then start adding the verbal "leave it" to the exercise. When puppy seems to be really understanding this, you can vary the exercise and start using moving objects (toss a toy, etc). I did a major reward for ignoring a toy thrown by him, so Johann would know that no matter what he ignored- there was something even better coming. You can use food reward or a play reward for this (we did a lot of tug).
Now, you can start using the "leave it" in relation to chasing the cat (this didn't take Johann long to get to this point- maybe a few days to a week). Keep puppy dragging a leash and let the cat into the room. If puppy doesn't immediately chase- click/treat. Continue to click/treat any time the puppy looks at the cat and doesn't react. When puppy does start to move to chase, say "leave it" and start to walk in the other direction. Do a click/massive treat when he turns away from the cat. Once you feel like the puppy as figured out "leave it" means to not chase the cat- you can remove the leash. Also, hopefully by this point your cat is more comfortable and less likely to run away. By the time your puppy is old enough to be left out of a crate or xpen when unsupervised, you shouldn't have to worry about him chasing the cat.
It does take a lot of work, but you can end up with a boxer and cat that live together with no issues or concerns (in most circumstances).
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