CyberGene if you want to completely prevent Google crawling the users, you should also add a DISALLOW rule in your robots.txt file. The setting in Sitemap stops advertising the profile pages in the sitemap, but otherwise doesn't discourage the search engines to find them via other means.
The fact they are absent from the sitemap might trigger some logic in some search engines, but I don't think there's any industry standard to say search engines should ignore pages not listed in the sitemap.
I wasn't aware the priority had an influence on the ranking of pages in the search results. I was under the impression it helped the search engine choose in which order to crawl the pages. That's why the number is higher for tags, because there are usually very few tags pages and they offer a lot of cross-links to other recent and important pages on the forum. So it would make sense to start from there, and only later go through each discussion on the forum one by one.
