Hi. We have been approached to open a business account for a bitcoin mining business (which is also a foreign based business). We want to turn them down, but we don't want to get into any trouble regarding de-risking. How can we safely approach this/respond to them to tell them no thanks?

That is not de-risking - just tell them no.

You aren't obligated to open an account for everyone that walks through the door. No works just fine.