There is an codeproject project avaliable HERE that provides this functuanility.
Following many threads here on SO and other boards this cant be done with the normal MessageBox.
Edit:
I have an idea that is a bit ehmmm yeah..
Use a timer and start in when the MessageBox appears.
If your MessageBox only listens to the OK Button (only 1 possibility) then use the OnTick-Event to emulate an ESC-Press with SendKeys.Send("{ESC}");
and then stop the timer.