To stop the execution of the program from user exit

Hi folks,

I am using a user exit for the Tcode ME51N. I am displaying a pop up box for the user to confirm a decssion. There if they press 'YES' system will continue and the transaction is saved. if they press 'NO' user has to come out of the Program and Tcode completely.It mean the trasaction will be cancelled and the user will be SAP intial screen..how can i do it from a user exit. pls suggest me.

Thanks in advance,

Shyam.