Should I just friggin hold the site hostage?

Okay guys, I need someone who dealt with this before to give me some advice on this.

I had a huge project to redesign from the ground up the Special Olympics Hawaii Website, not only that I have to add alot of new content and in the mean time keep their old site updated. Anyway, we agreed on 2 equal payment, one when about 50% of the new site was done, and the other when it was finished. Now I have about 75% completed, and long ago mailed the invoice.

I have emailed the invoice about a month ago, and I have received nothing from them, not even a confirmation no emails, nothing. We used to have weekly reviews from them on our progress (all positive btw). And now nothing.

Now I really hate to start threatening an organization like Special Olympics, but as it seems to me, and it looks like they arent being very cooperative.

Now one mistake, that I realise I made is; I only have a verbal agreement with them. Thats about it. Appart from the emails we have for communication and some of their requests and whatnots.

I can always take down all I have from their servers, including their old site and replace by an empty page saying : SPECIAL OLYMPCS HAWAII ARE CHEAP BASTARDS! As well as change all their passwords on the servers and not put anything down until they cough up the dough?

Anyway, have any of you dealt with something similar? Now I can understand that a greedy individual would do something like this, but an organization like Special Olympics? Anyway…

Have any of you dealth with something like this? Or something similar?