Personally, I think you should discipline the cat in a similar manner to how you would discipline a small child that throws a temper tantrum. Sure, you can’t reason with the cat, but you can show it that the behavior it’s exhibiting is not ok with you, the master (all kidding aside, cat owners who talk about being “owned by” their cats are somewhat scary to me. Parents who are “owned by” their children have spoiled brats for children. It’s no different with cats.)
At six months, the cat is still plenty young enough to learn how to behave. If you pick it up, and it struggles and hisses, and you let it go, you’re just teaching it that by throwing a tantrum, it can get what wants. Instead, when it strugges and hisses, try holding it gently but firmly, not allowing it to get away or to scratch you (admitedly, the latter can be difficult, but it’s doable). After it calms down, at least somewhat, only then let it go. By doing that, you are teaching it that struggling and hissing doesn’t get it what it wants (to be let go), but that being calm does.
I speak from experience here. I have a ten-year old cat that I got from the Human Society when she was a kitten. I picked her over some other kittens at the Humane Society because when I picked her up, she didn’t seem afraid, didn’t struggle, and just let me hold her (as opposed to some of hte other kittens I tried picking up).
But after I took her home, and some time went by, she started behaving like Rebekkah’s cat, struggling to get away when I picked her up. Far from taking the advice of not picking her up, I made a point of picking her up more often, and holding her until she calmed down. And please, no one jump to any conclusions here, I absolutely did not hurt her in any way, I just didn’t let her go right away.
After a while of that, she got the idea, and didn’t really struggle when I picked her up. Oh, it was clear that she wasn’t all that happy, and it’s not like I’d pick her up and hold her for hours or anything, but she accepted that struggling wasn’t the right thing to do.
And you know what? After even more time, she got to actually like being picked up and held, at least some of the time. For a while we had a ritual that when I got home after work, I’d pick her up and prop her front paws on my shoulder and walk around a bit, and she’d sit there happily and purr.
Now, ten years later, there are still times when I pick her up and she sits and purrs, and other times when I pick her up and she’d clearly rather be down. But she still doesn’t struggle, and I respectfully put her down in short order. She is an extremely gentle, loving cat, and I believe that at least part of why she’s so gentle is that I taught her, early on, that struggling, and clawing are not acceptable.