One thing I find weird in the Unfinished Tales description of the War of the Elves and Sauron is this snippet from the conclusion: “Sauron was routed utterly and he himself only narrowly escaped. His small remaining force was assailed in the east of Calenardhon, and he with no more than a bodyguard fled…”
So Sauron, utterly routed in the battle, fled with a bodyguard (what would actually constitute a helpful bodyguard for a Maia) and only narrowly escaped the field.
And I just think that’s such an odd thing to emphasize because, like, what if they'd caught him?
Because as far as I can understand it, Sauron can take a physical form if and when he feels like it. My initial instinct was that capturing him wouldn’t be possible at all, but Tolkien probably wouldn’t have written that he narrowly escaped capture if it was metaphysically impossible.
(Lúthien captured him. But she’s a Maia herself, so I’m not sure this tells us anything about how plausibly Elves could have done it.)
(The Numenoreans captured him but only because he let them.) But I really seriously don’t think Gil-Galad’s that stupid. For that matter, if they capture him, decide not to do the stupid thing, and execute him on the spot, does that make any difference? As long as the Ring exists, he can’t be destroyed (and they don’t even know this, so destroying the Ring wouldn’t occur to them) and he can reconstitute his body at will, right?
So now I’m thinking that maybe the process of shedding/changing a fána is really slow. It’s referenced casually in Valinor, as something that the Ainur do for festivals, but time passes differently in Valinor - if it took five years to take physical form by the Middle-earth count of time, you could still do it for festivals in Valinor without particular difficulty. (It could also just be easier in Valinor because Valinor is better suited to the Ainur way of processing the world.)
And if your old fána were destroyed violently, that would make one averse to creating a new one, and with the Valar aversion-is-inability-is-experience. So perhaps getting violently killed makes it take even longer.
All of this would combine to suggest that, if you kill a Maia in Middle-earth, that’s plausibly a real and damaging setback to them. They can assume physical forms at will, but they aren't shapeshifters and this really isn’t either a combat ability or a form of effective invulnerability.
After I’d thought through all that I realized I was an idiot and there was a much simpler explanation.
Sauron had the Rings on him. And, sure, if they’d caught him they could kill his physical form and put him out of commission for a while, but significantly more important is that they’d have gotten all the Rings back (and even if they didn’t know what the One Ring did, they’d have had a lot of time to figure it out.) And that’s almost certainly why Tolkien bothered to mention that he barely escaped capture - whether or not capturing/killing Sauron would have mattered to Sauron much at all, losing the Rings would have been devastating.