You have to open the Developer Tools → Sources tab to see the mining web workers. They will be named like "#1", "#2" etc. with small gear icons next to them.
What NoCoin does is that it closes them before they could work, and the website reopens them - it's an infinite fight.
Just look at Opera's CPU use in the Task Manager and you should see.
Unfortunately NoCoin doesn't seem to work for ThePirateBay's WASM (assembly) scripts at the moment.