[mme42]

Yes it's a phrase called "first_visit_message." You could search in the phrases for some of the text ("If this is your first visit" for instance). Or, you could scroll through the Global phrases alphabetically until you come to "first_visit_message." That's if you want to edit it to something else.

If you wanted to remove that box completely (and instead use a notice for instance) you'd edit your 'forumhome' template and remove the code between the <!-- guest welcome message --> comments.