This post is a little old, but since it took me a while to figure it out myself, I want to post that you can get all posts by a user by using the search feature. On the tool bar directly underneath the navigation bar of the forums, click "search". The search page will come up and there will be a search form to fill out. Scroll to the option that says "search using filter" and select "author" from the drop down menu. Then, type the person's username into the "keywords" text box at the top of the search form. To get every post they've made in any forum, you need to make sure "Search All Forms" is selected in the drop down box that says "Search which field(s)?" and "Search All Topics" is selected in the drop down that says "Search how many days in the past?."