Do I have right to ask a client to pull down their website?

[Frustrated]

I have created a series of Facebook Apps and a website for a marketing agency for a couple of their clients, however we are now in dispute regarding some of the invoices which have been raised for the work completed. The client did not ask me to sign a contract / agreement and as such I believe I own all the custom source code which was used for the apps and website.

What rights do I hold on this source code? Do I have a right to tell them to pull down the website and apps unless that pay what is owed?
Update: I should have added I am waiting on a callback from the Copyrights team at IPO. Thought it was worth asking for some public opinion while I wait.