idbeholdME: Make it 2 or 3 instead of 5. Make them jump through extra hoops. Captcha for every thread after you make 2 or 3 per day. E-Mail verification where you have to click something for the thread to be made if you make too many in a single day. Anything really. But letting them run rampant is not optimal I'd say.
Bookwyrm627: All of that gets avoided by a spammer making multiple accounts (and I've personally seen the multiple accounts thing happen before). At the same time, all of these solutions could easily inconvenience legit users, which is definitely something to avoid.
To be clear, I don't object to stopping spam. I wish to point out that "stopping spam" is a much harder problem to solve than one might think. It is the proverbial arms race.
Currently, the guy just had to create 2 accounts, start a script on them and the flood was here. If he had to create 30 accounts to create the same amount of spam, he might reconsider if it is worth his time. Given that currently, there is exactly 0 things that would at least in some way annoy the attacker, there is no reason for them not to spam.
A start would be for example, that new accounts could create only 1 thread per day. This would increase the longer the account is active.