Nice question. I think Morgoth
could have been physically killed - after all,
Myths Transformed tells us that the Valar did exactly that to him after the War of Wrath; now Beren wasn't a Vala, of course, but he was in the perfect position to at least try it. So why indeed didn't he just slit Morgoth's throat instead of fiddling around with the Silmarils?
I have a hunch that the crucial point wasn't ability, but ethics - as in, the Eldar (and the Edain by extension) probably didn't consider it right to kill a helpless foe under any circumstances except in lawful execution of a condemned criminal (like when Eöl was taught bungee-jumping without a rubber band). Impractical to the point of foolishness when applied to someone like Morgoth, maybe, but that's just how I'd expect these people to tick.
Beren may also have felt something like Frodo did when he said about Saruman:
Quote:
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'No, Sam!'[...]'Do not kill him even now. Fo he has not hurt me. And in any case I do not wish him to be slain in this evil mood. He was great once, of a noble kind that we should not dare to raise our hand against. He is fallen, and his cure is beyond us; but I would still spare him, in the hope that he may find it.'
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(Emphasis mine)
If there was little chance of repentance and cure in Saruman's case, it was converging on zero in Morgoth's; but the chief point of comparison for me is a mortal's reluctance to kill an Ainu, due to the feeling that a being of higher order should be dealt with by their peers (or in Saruman's case, superiors) - i.e. the Valar; and note that the host of Aman didn't kill Morgoth on the spot either when he was taken captive, but brought him before the Valar to be tried for his crimes, condemned and executed.