Well there is no option in the component to do this. So you could change the source code and use message/notices for joomla.
Check:
http://docs.joomla.org/Display_error_messages_and_notices
But this would change a lot because all validation is done by javascript. So you must change the whole validation process.
The mathquestion answer can be found somewhere in a piece of javascript. But this is not a problem because spambots arent that smart. They wont recognize this number from other numbers in the source code of the script.