What to do if someone has locked their bike to yours?

For that sort of money you can get your hands on a set of proper fuck off bolt cutters, the sort you see being used to cut d-locks etc. Quicker, quieter and more reliable. And you won't have to replace the cutting disc every time either.

What bolt cutters can get through a decent d lock?

On the almax demo's it shows the biggest possible cropper being used to cut chain which is thinner. It looks like the jaws get a bit fucked in the process too.

Angle grinders can cut through everything, even Krypto Fahg's