How do you change or get rid of a timestamp server?

I added a timestamp server not really understanding what it would do. However, Adobe Acrobat X still tries to use the original timestamp server even after blocking the site. When I first tried to use the timestamp, I was prompted to set up a server and make it the default server before the timestamp would work. The prompt no longer appears (even when original site is blocked), and instead, I am prompted to allow the original site or the timestamp doesn't work at all. How do I get back to the starting point or at least change the server?