Traian, a member at SEW, is asking for help in what to do regarding a client who will not pay for a verbal contract which he completed. The discussion is fascinating as both ethical and legal issues arise. But, in the end, Ian McAnerin chimes in with a great post on the subject:

I've excerpted a subsantive bit as I believe it's important for every small business owner or independent contractor to read (regardless of profession):

You DO have the right to:

1. Remove all your changes
2. Since there was no contract, you are independant and they have not paid, you OWN the changes and therefore they are your copyrighted material (assuming they were copyrightable) - if they use them without your permission, you can send DMCA notifications to the search engines and their ISP to have their site removed. This would be especially egregious if you removed them and they added them back afterward.
3. In many jurisdictions, you can place a lien on a website (similar to a mechanics lien on a car). It would be best to get a court order for it, but many jurisdictions will allow you to do it informally if you have control over it already. Change some passwords then put a big sign at the top saying that the website has been siezed for non-payment. Check with your local laws FIRST. Some jurisdictions will not allow this without a contract saying you can do so signed ahead of time.
4. You can sue them for unjust enrichment and/or common law fraud, which is intended to cover this type of thing. It's not an easy win without a contract, however - but it does work if you have full documentation.

Ian also notes that:

A principle of natural justice (on which the common law is based) is "he who seeks justice must also do justice"