Background

They paid me my first half (back in April 15th) before even we could get into an agreement. Very nice of him! Then I've finished the 2 weeks job of setting up the servers, using his AWS credentials he had just bought. I waited for another 2 weeks for everything settling up, and it was all running fine. He did what he needed with his sftp account, everyone were happy.

Now, it has been almost 2 months since I've finished the job and I still didn't get the 2nd half. I must assume, it's not much money (about U$400, converted), but it would help me pay the bills at least. Heck, the Amazon bills they are paying are little less than that (for now).

Measures

I'm wondering how I can go to charge him now. First thought, of course, would be taking everything down and say "pay now, or be doomed". If that's not good enough, then I lost it. I have no contracts and I doubt I could get a law suit in this country for such a low value based only on emails. And I don't really want to get too agressive here - there might be a business chance in the future and I don't want to ruin it.

Second though would be just changing the password. But then he probably could gain access again by some recovery means. That's where my question may mainly relay. How can I do it and not leaving any room for recovery from his side? I even got the first AWS "your account was created" mail from himself, showing me I could begin my job, back then.

Lastly, do you have any other idea on what I can and what I should do in this case?

Responding to Answers

Please, consider reading the current answers and comments. This is not a very simple case. I've considered many, many options (including all lawful ones) before considering this ones I've listed here, and I am willing to take the loss and all that. That's not the point.

The point is being practical here. I will call him again and talk about it. I will do terrorism on getting lawyers and getting contract. I am ready to go all forth while I have time and energy for it.

But, in practice, there is this extra thing I can do to assure myself of the work I've done. I can basically take it back and delete everything! I'd only take his password because I can find no other way to do it within Amazon. Maybe, contacting Amazon and explaining the situation? I don't know. Give me ideas on this technical side!

And thank everyone for the attention and helping me clarifying the issue so far! :)