Up until today I've allowed commenting in my journal from everyone and have not turned on IP logging or screened comments. Today I've received two comment spams from two different registered users. I banned the users and reported the comments as spam. To prevent more comment spam I will be turning on comment screening for people not already on my friends list and IP logging for all comments. I'll probably turn it off again in a little while in the hope that this is a one-time problem.
What a fucking bore.
ETA: Screening appears not to be working in this journal, although it's working in my free account. I've put in a request to LJ support, but if anyone else has had this problem, help would be welcome.