Before I begin I want to address the word "spying." Despite some of the more hyperbolic headlines and articles you might have come across, there is no evidence to suggest that Microsoft is using this data to actively spying on anyone. The data that's being collected is being used to make Microsoft products and services better, and crazy claims aside, no one has been able to produce a shred of evidence to the contrary. This is why I've put the word spying in quotation marks in the title, and I'm only using this word because this is the word most commonly used by those concerned by this issue.
If you ask me if you be worried about using Windows 10, my answer would be a "no." I have several Windows 10 installations here and I'm not concerned.
But despite such reassurances there are a lot of people who are concerned by this, and the fact that Microsoft isn't willing to give concerned users a way to opt out from data collection (which I think is a bad idea) is adding fuel to the flames.
Another problem is that all this hysteria is making people go out and look for tools and utilities to put a brakes on Microsoft being able to collect data from their PCs, and some of the tools out there are, well, shady at best. I've come across a number of "Windows 10 privacy tools" from unknown sources that do who knows what. Some tools actively display ads, and one even installs a third-party tool that displays ads in other applications.
Talk about taking what is a non-issue and blowing it up into a real problem! No self-respecting privacy tool should install adware onto a system. Period.
If you really need to block Windows 10 from sending telemetry data back to Microsoft then the utility I recommend you use is Spybot Anti-Beacon. It's a one-click solution (along with an undo button in case things don't go as you planned) from a known developer that's been in the privacy business since 2000. And there's no adware!
I've tested this tool on a number of systems and it seems to do what it says it does on the tin, and nothing more.
I want to reiterate that I don't believe that there's a need for anyone to do this because there's no evidence to suggest that Microsoft is doing anything evil with the data. But if you just want to opt out, this is a safe way to do that.