Connect both machines to the router, and make sure they both have received an IP-address (DHCP).
Log in into the router via your favorite admin password, select the "Advanced" tab on top and then select the "Access" subgroup on the left.
Now enable (checkmark) the access control, Add Policy, and follow the wizard.
1. Define a name for the policy (like "Block <machinename>")
2. Select "Always"
3. Select MAC Address-type and select the machine from the pull-down next to "Machine Address"
4. Click OK to "Load" the MAC-address and click NEXT to complete the list of blocked machines
5. Select the "Block All Access" method and click SAVE
You should now be back at the Policy-overview with the machine in the Policy-table with a checkmark. Click on "Save Settings" and Internet access is now blocked for that machine.
If you need to temporarily allow Internet Access for the machine (like when refreshing the Antivirus-definitions, or loading updates to the OS), then simply remove the checkmark, "Save Settings". Afterwards just enable the Block via switching on the checkmark, and "Save Settings".
Please note: The "Save Settings" is very important, and the checkmark should be verified...
Happy hunting...