I'm not a doctor and I have no idea what I'm talking about, but to my (very limited) understanding of the mechanics of it, it's not that simple.

You don't get/stay erect because of the flow to your penis, sure, that's an important part of the process, but the key to getting/staying hard is blocking flow out of the penis. In theory, again, I have no actual idea, I'm just speculating, you could go in and shunt the vein to force it to open up all the way to drain the blood, however, you may hit an obstacle in trying to do that, like a titanium cock ring.

From the limited amount I know, it's possible to drain the blood directly, but my understanding is that it's not a very fun thing to try to do, because as soon as you lose the erection, it opens up vessels and more blood can flow in, so it turns into a bloodletting through your cock, and you have to essentially let the blood out faster than it can flow in, unless you cut off the arterial flow into the penis, which isn't as simple as a single clamp. The arteries are also inside of you, so that's also a problem. Fun times in the surgery bay? (Not really)

The whole thing is far more complex than I really understand, so I might be way off. I'm no doctor and I'm certainly not a reproductive organ specialist (or any adjacent discipline). I'm certain that someone with far more time spent in school who earns way more than I do, can correct me on every point here, so I'll stop.