Great points in this thread.
First of all, they shut me out of their servers, with no warning, otherwise I would've reverted their site back to where it was, at the very least.
Second, I do White Hat only, so I'm not too sure that those SB blasts would affect their site, but maybe you have evicence of the contrary.
And yes, I think that being vindictive is sometimes a moral duty, just to make sure this doesn't happen to other SEOs, it's not just about my pride, I swallowed in the past in other situations, but I didn't make the world a better place.
So here's what I am considering:
Website in the exact same niche; with so much research work done, I'm not going to waste it, I'll see how to monetize it, but I am surely going to beat it no matter what SEO they hire (I can tell you why if you want, but it's boring).
Threaten Small Claims Court.
Contact their competition, especially one that I know they hate and help them.
Register theirwebsitesucks.com.
Damage their reputation where I can, including Facebook, LinkedIn and wherever the person I dealt with is.
Copy their content and post it somewhere strong (suggestions welcome) hoping that Google sees their website as a duplicate.
Complain with their host for keeping and using my code tweaking SEO work.
Anything else?
PS: They probably are not aware of the Golden Rule: DO NOT screw a SEO (or a geek, whatever)