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?
:bad:

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?