How to disable "reject call with an SMS"? ICS introduced an option to reject incoming calls with an SMS.

Is there a way to disable this function. If yes, how? 

Sony Xperia S handset running JB
Unfortunately, there is no way to disable that. Sony is using a proprietary Dialer that has that function hard-coded into it. Without using a custom ROM, there is no way around it.

I know this because I have experience with this on Samsung devices (they also have a bar near the bottom that allows to Reject with Text), and having heavy proficiency with Android, outside of somehow recompiling the Dialer apk and using your knowledge of Java to find the entry, remove it, and recompile, there is unfortunately nothing you can do.