In France, we 've contact Alcatel to know if we can delete the possibility for a user to push ''stop ring'' for an incoming call...answer is negative. Have you got a solution somewhere ?
What version of software did they introduce that feature? Maybe you could consider going back to a version that does not have it.
I'm guessing that the feature is being abused
If so, another option is to see if your customer will put out a company wide email informing staff that usage of the feature is being monitored. A bit of physcology might be enough