Since Netflix is just a website, you can block it by adding the following domain under the website tab when editing a block list:
netflix.com
On Windows 10, the Netflix app is unfortunately tightly integrated within Windows and can't be blocked normally. You do have a couple options though:
1) Block all Windows 10 style apps, by adding these exes to the block list:
C:\Windows\system32\applicationframehost.exe
C:\Windows\system32\wwahost.exe
2) Uninstall Netflix and block the Windows Store app so that you can't reinstall it. The Windows Store app is listed under the Win 10 tab as: WinStore.App.exe