0) Log into steam in your primary browser. 1) Click on the offender's name in chat. 2) Click on "profile" > "steam" (this launches profile page) -- You probably also want to click on "block" in this menu, probably before reporting 3) Click on the "More" button on the right side of the profile header, next to "Add Friend" 4) Click on "Report" I would select "spamming ad/links" or "harassment", and then type in a comment about spamming Card Hunter world chat several times per second for several minutes.
