I have a couple thoughts for you:
Make sure that your database is creating pages that are crawlable by search engines.
Create unique content for those pages that are optimized for keywords and other SEO best practices. Especially check what HTML your database is spitting back out, not all CMSs are created equal.
As far as what to do with those job pages after the position is filled, that's really up to you. I do think that people expect filled jobs to disappear into the void. Though you could put a "FILLED" announcement on the page and create something where it would recommend unfilled jobs that were similar. On an e-comm site I worked on, we decided to leave up sold out product pages, but make it super clear they were sold out (and of course, they were not linked internally anymore), due to the traffic coming from search engines who'd crawled those pages and external links. Then the pages had a "recommended products" on them which drove people to similar items.