Hang on !
I would definitely avoid "301 back to the root page for jobs" or even a category page.
Over time, you are going to be creating a massive index of empty pages linking to a home page; that looks too spammy to me. If you want to be honest : 404 these pages- the job offer no longer exists, the page no longer exists --- you can personalise your 404 page to send the user to a relevant page
Honesty doesn't always pay though! To leverage the SEO benefits from these pages I would consider archiving the job listing, keeping the same url and just adding a message indicating that the post has been filled (an image will do)
That way, you’re keeping lots of unique content on your site and over time creating a log of pages.
To make these archived job pages useful to the user and to the search engines, dynamically add links to fresher job offers in the same category, company and town.
- Neil
PS Does this new SeoMoz feature now mean I'm now paying to give free advise ?