#1 would be the most logical of the 3 but I am in agreement with Bluedreamer.
If there is no form until the user clicks the ‘add’ button then simply do not display the button but a message saying that comments are no longer being submitted to this article.
We do, and it’s horrible design, because it doesn’t make any sense, and the difference between open/close are too small.
However that button does serve a purpose when a thread is closed because mods can still press that button we a thread is closed and still reply. (not that that is any help to non-mods, but okay).
I agree with Remon. If the comments are closed, there shouldn’t be a button to push - especially one which has a hover state which implies something will happen if you click it. Not to mention the accessibility/usability issues which will occur with keyboard users and having to tab through a useless button.
Not quite sure why you’re worried about space/layout. If comments are closed, don’t show the button and have the test in exactly the same position - problem solved.
The message “Comments Closed” is perfectly adequate, but as it is not a button, don’t style it to look like a button. (In the same way that you shouldn’t underline text which isn’t a link - unless your aim really is to confuse folk. ;)) Put a border round it or something, if you feel you want it to stand out, but I don’t think that’s necessary. Anybody using your site will know there’s an “Add Comment” button, and if they go to look for it in the usual place and see instead a plain statement “Comments Closed” then they’ll quickly get the message. On the other hand, if they see a blue button where they expect to see a blue button, there’s every chance they’ll click it without paying much attention, and get frustrated that nothing’s happening before they notice the change of text.