This may not be what you wanted to hear but AFAIK there is no way to do such thing, you can disable the headset from being used and also the speakerphone leaving only the handsed.
but those disable the whole feature, there is no way to disable only the button from the phon if you want to use it.
in this case the matter is more of educating the agents not to do that, since CUCM is unable to do so
maybe some super glue on the button?? =D
HTH
HTH
java
if this helps, please rate