How do I stop the ADVERTISEMENTS that show up while watching the stream?

Yes, Ustream has ADs, it is something that we can somewhat control by installing an add-on called: ADBLOCK +
Click the link for the corresponding browser you use to install the add-on.

ADBLOCK + for Firefox
ADBLOCK + for Chrome

Here is a tutorial to make sure you've setup ADBLOCK + correctly: ADBLOCK + Setup TUTORIAL

Finally, here is a link to EASYLIST, which allows you to disable the ADs in different languages.