Q. Is there a Group Policy to block the Windows Store?
A. Yes. Group Policy can be used to block the Windows Store however this policy only works with Enterprise and Education editions of Windows 10 after the 1511 (Fall) update. This policy is ignored by other SKUs of Windows 10. To configure navigate to Administrative Templates - Windows Components - Store - Turn off the Store under either User or Computer.
Under the same area of group policy is also "Only display the private store within the Windows Store app" which also only works for the Enterprise and Education SKUs of Windows 10.
One way to block would be to block the use of Microsoft Accounts which is required to use the Windows Store (outside of Azure AD accounts) however this would impact other features such as Cortana.