If it executes, it can be decompiled. Objective-C is particularly easy to decompile into readable code thanks to its dynamic features. If you want to make things a little bit harder, you could write most of your code in plain C and strip the executable — which of course has the tradeoff of making your application design harder to manage. But be honest with yourself: If somebody wants to crack your code, you are not going to be able to stop them. Crackers have practically unlimited amounts of time and enthusiasm and will actually be excited by any novel efforts you put in to stop them. Nobody has yet made uncrackable software, and the biggest corporations in the world have tried. You’re not going to do better than them, especially if you need to ask about it on Stack Overflow.
Take the time that you would have put into thwarting decompilation and use it to make your product better — that will have a much better ROI.