Westin: Go to the site that needs a custom on/off setting. Right click on page. Select "Edit Site Preferences...". Click on the Scripting tab. Tick/Untick "Enable Javascript". Click OK to leave the dialog.

Alternative: Use the "quick preferences" menu (F12 on windows) to toggle JS on and off as needed. 

Opera also supports UserJS scripts and compatibility for GreaseMonkey scripting.