While you can disable it browser by browser, you may wish to just solve the problem system-wide, like so:
Open Java Preferences<br /><br />
Use spotlight to open the "Java Preferences" application and stop at the first tab, General. Then proceed with the optional disablement steps below.
Disable web browser plugins system-wide (optional)<br /><br />
Uncheck "Enable applet plug-in and Web Start applications."
Note: You will need to restart your web browsers for the changes to take effect. A good site to test against is http://pingtest.net - with Java on and the applet accepted it should be able to measure dropped packets, with Java disabled it should not be able to do so and will tell you as much as you run the test.
Disable Java applications, applets and command line tools (optional)<br /><br />
Uncheck each Java instance you would like to disable.
Note: Start/restart and Java applications you might like to test this change against.