Should I just shell out the 25bucks for a flip-flop hub or fixed hub and build a wheel around that?
Yes. There may be a way to disable a freehub. Like pouring sand into it. But really, when you read your description of how you're planning on making a rear wheel that you will at least partially depend on to stop, does your idea sound like a good one? I am the world's cheapest bast@rd, and I would not do that.
In fact, if I were you, I'd shell out the $50 and get a built fixed wheel. I built my set of fixed wheels, and have pretty much regretted the extra $150 ever since.