How bad is the relationship already?
Personally, I wouldn't burn a bridge with a client who could either return later or pass along some word of mouth, for $1200. If the relationship is already spoiled to the point that you don't think either of those things might happen, I would just explain to the customer that you can't afford to do any more work without payment, and then walk away. Don't retaliate/do additional work to turn off the site or whatever, just literally walk away from it as is.
If you still have a decent relationship, I would just explain to them that you have $1200 into the project, and you understand where he is coming from, but would really appreciate it if he could do better by you. If he returns with a negative, say thanks, and please pass along any other opportunities, give him whatever access he needs and walk away.
Retaliating in any way isn't going to get you paid, it just will spoil your reputation, which is worth alot more than $1200. Even if you are right and he is wrong, if he happens to be at a bar with someone and that person mentions they are having you do some work for them, whether he responds with "Oh yeah, he is a decent guy" or "Oh man, did he burn me once" could make a difference.
Life lesson the hard way.