Navigator 3.x 
Go to "Options | Network Preferences..." from the main menu
Change to the "Languages" tab
To enable: make sure "Enable Java" check box is checked
To disable: make sure "Enable Java" check box is unchecked 
Restart the browser

Communicator 4.x  and newer
Go to "Edit | Preferences..." from the main menu
Select "Advanced" panel
To enable: make sure "Enable Java" check box is checked.
To disable: make sure "Enable Java" check box is unchecked.
Restart the browser

Explorer 3.x 
Go to "View | Options..." from the main menu
Change to the "Security" tab
To enable: make sure "Enable Java programs" check box is checked
To disable: make sure "Enable Java programs" check box is unchecked 
Restart the browser

Explorer 4.x 
Go to "View | Internet Options..." from the main menu
Change to the "Security" tab
Select "Custom" and click on the "Settings..." button
To enable: make sure "Disable Java" is selected under "Java."
To disable: make sure a setting other than "Disable Java" is selected under "Java." If you're not sure which setting to choose, select "High safety"
Restart the browser

Explorer 5.x and newer
Go to "Tools | Internet Options..." from the main menu
Change to the "Security" tab
Click "Custom Level..." button
To enable: make sure "Disable Java" is selected under "Java."
To disable: make sure a setting other than "Disable Java" is selected under "Java." If you're not sure which setting to choose, select "High safety"
Restart the browser

Simply reloading the page, rather than restarting the browser, maybe sufficient under most conditions and browser versions.