Lol, with all respect, taking down a companies website even with credentials given to you (for another job) can be construed as hacking. Revenge hacking can get ugly pretty quickly (criminal offence). You have a contract for X, do X and don't get paid for X. The conflict over paid for X is in a whole different ballpark than illegally using credentials for Y (taking down their site, with obvious damages as a result).

I was more referring to the $30k example above. In your situation probably fine for a few thousand, but if someone owes you a significant amount of money then I don't think you should act in this way. You just sue them (if all else fails).