In general you will have to decrease the number of links in your menu. Don't think that a choice of 600 links is really user-friendly. But of course that might be subjective. We have a client with about 300 well-reasoned links on menu and the site performs quite well; most of the pages have great rankings. The SeoMoz crawler still finds it inappropriate but the interaction and the rankings are good. Google are saying that you can have up to a 100 links and that is considered to be normal. Check amazon. They don't have more than 100-ish links on a single page. Think about it and try to cut the bulk from your main navigation if possible.
You can always put no-follow links to the subpages and pass seo juice to the main categories and subcategories only. That's the easiest way to solve the task but nothing will change for your users.