[Misiek]

You can change an action of the form using Javascript (ie. http://stackoverflow.com/questions/2701041/how-to-set-form-a...). Spam robots don't emulate Javascript. Users with no Javascript will see the message "This action requires Javascript".

[Prefinem]

The problem is you would lose the ability for users to post this way. Seems strange to cut out a percentage (albeit probably very small) of users to stop spam