I keep getting to the final interview stage with pitches and ideas, only to be rejected and then see that my pitches are being used by the companies in the following months. Does anyone have advice for this? I want to shine and have great ideas but I feel like i keep getting rinsed to work for free and then be let down.

That's enraging. I wish I had some advice to give, but I really don't, unless you're giving those ideas in some sort of take home project. If that's the case, stop doing them. Or at minimum make them pay you for the time. No one should be working for free.

If your ideas are truly original and they are being stolen, and you have time stamped emails showing the work you shared, there has to be some way to hold them accountable. If the ideas are broad or obvious that anyone may land on them, different story, it’s hard because that could be subjective but I’m irate on your behalf that they are doing this! Is it an agency or a brand?

Companies typically ask people to submit things just because they need to see some work samples. So it's a dilemma, if you want a job you need to participate in that sort of exercise. Accusing companies of stealing your ideas is a serious charge, and if you have proof of what's happened you should be talking to a lawyer. It might be a challenge, of course. That's why some publishers and film companies refuse to look at any work that's unrepresented by a credible agent, they don't want to be accused of plagiarism and have to deal with litigation.

The company should provide a dummy version of a project that has no relation to their company so they can see how you think and approach challenges vs trying to solve their problems in a take-home. As another said here, you should also, when confirming the assignment, send them your hourly wage so they can pay you for your effort. If you’re getting to finals, that’s a great sign! Hang in there! The “yes” is coming soon!!

That sounds really frustrating, and I’m sorry you’re going through that. It’s great that you’re reaching the final interview stage—that already shows companies see strong value in your ideas and thinking.
One thing that might help is keeping your pitches more high-level during interviews. Focus on the strategy and your approach rather than giving away the full execution details. That way they can still see your thinking without you essentially doing free consulting work.
You could also frame ideas as examples of how you would approach the challenge rather than a fully developed plan.
Unfortunately this does happen sometimes, but consistently reaching final rounds means you’re clearly doing many things right. Hopefully the right company will recognize your value and bring you on board.

I would tell them what would i do but not how to