If you are using Xperia E1 model then you cannot disable reject call with message option in anyway. Even in the pre stored msg template you cannot do anything. You cannot even delete either of those messages.
If you are having problems in receiving call i.e. if your calls are auto rejected because of pocket movements then the only way for you is to use a small app called as "Tiny Call Confirm" which you can easily download from Google playstore. This app will lock your screen when you will receive a call. This app is small in size just a few KBs. and it works perfectly for every incoming call. Many users are facing that issue.
