How to block all traffic but one IP in Windows Firewall?

The proper way to accomplish this is to configure Windows Firewall to block all outgoing traffic by default, and then only allow the incoming connection(s) you want.

To do that, click on Windows Firewall with Advanced Security in the left pane, and choose Windows Firewall Properties from the right pane. Next to Outbound connections, choose Block. Then, click OK.

Once you've done that, just delete the block all outgoing traffic rule and you should be all set.

You can do this with IPSec.

1. Click Start and type MMC then select the MMC from the list.
2. Click File -> Add/Remove Snap-In -> Add the IP Security Policy Snap-In and Click Ok
3. Right Click on the middle section and click "Create IP Security Policy"
4. Follow through the first wizard with the defaults giving the name/description you want.
5. With the Windows that opens after the first wizard click "Add..."
6. Click "Next" x3 and then click "Add..."
7. Give the name of "Block All" or something similar. Click "Add..."
8. Run through that wizard keeping all the defaults and click "OK"
9. Select "Block All" and click "Next" and then "Add..." again
10. Follow the wizard giving the name of "Block" and select "Block" as the action.
11. Repeat Steps 5-10 selecting the proper source/destination/protocol and selecting "Permit" in Step 10.