Use ajaxSingle attribute or a4j:region tag.
I tried ajaxSingle="true" on the a4j button. It doesn't work. Even region doesn't work. User guide says
ajaxSingle - submits onle one field / link. Can graphicImage be considered as a field of a control ?
Anyway, I got it to work by using rich:paint2D with minor modification to a good example here - http://www.emforge.org/wiki/ImplementingCaptchaWithJSF
But I think even my code snippet should work. Will ponder over it some more.
RF submits the whole form by AJAX. Region/ajaxSingle are the ways to limit processing on the server, but data is being send anyway.