Why do you want it disabled? Do you only want users to enter in a message of a certain type? If so, see the usage and pattern fields.
https://www.wowace.com/projects/ace3...bles#title-3-2
/edit: even if you disabled it, they could always hit "Enter" instead of clicking it.
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"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff,
The Tao of Pooh