There is some options:
1. you can't delete topic (first post) directly, you need to go to forum view - this is the reason why forum breadcrumbs are so important
2. you can delete latest post from admin area - not tested, maybe it deletes just post, not topic itself
3. with forum extended plugin you can delete topic if point 2 fails.
Yeah, this was spam post, but this is real user account. So did they insert it manually or their password was broken?
Nevermind, password was changed, user stays. I wish for "bad carma" functionality I am used from other CMS to track any activitiy of selected user.