How do I block an unknown device from my network?

Hi,

1. What was the recent hardware and software change made to the computer?
2. Are you using a wired or a wireless connection?

Please follow the links mentioned below to help troubleshoot the issue.

Method 1 

Please follow the suggestions mentioned by our Support Engineer  Gokul T and check if it helps.

http://answers.microsoft.com/en-us/windows/forum/windows_7-networking/unknown-and-large-question-mark-on-my-network-map/251bac71-31fb-4d22-9265-b1770ba8b7ee

If you still face issues, please check your adapter settings as mentioned below.

Method 2 

Uncheck Power management to your Network adapter 

To turn power management features on or off

a. Open Network and Sharing Center (click the Start button, type Network and Sharing in the Start Search box, and press Enter).

b. Click the Change adapter settings link in the upper left of the navigation pane.

c. Right click the network connection you want to enable/disable power management support on and click Properties.

d. Click Configure.

e. On the Power Management tab, check or clear the Allow the computer to turn off this device to save power check box.

When checked, power management is enabled on the network adapter.

When cleared, power management is disabled on the network adapter.

f. You can enable Wake on LAN for all wake methods or just enable magic packet WoL:

To enable Wake on LAN for all methods, check the Allow this device to wake the computer check box.

Hope this helps.