That's a very fundamental conflict when implementing these types of anti-spam measures. Unfortunately anything that is screen reader friendly is often spam-bot friendly, (or, anything that is unfriendly to spam-bots will also be unfriendly to screen-readers) as both entities are programs that attempt to parse a web page.

Maybe include a link near the start of the form that says something like, "Screen-readers, follow this link."? The link would be to a one-time use page that is sane and accessible. I don't know...