If you can't add a static ARP entry from an elevated command prompt with the ARP command, you can do it with the netsh command instead.
#Failed
C:\Windows\system32>arp -s 192.168.12.80 08-f1-ea-95-a1-2a
The ARP entry addition failed: Access is denied.
# Find the interface name
C:\Windows\system32>netsh interface show interface
Admin State State Type Interface Name
-------------------------------------------------------------------------
Enabled Connected Dedicated Ethernet
# Add the ARP entry.
# netsh interface ip add neighbors "InterfaceName" "IPAddress" "MACAddress"
C:\Windows\system32>netsh interface ip add neighbors "Ethernet" "192.168.12.80" "08-f1-ea-95-a1-2a"
C:\Windows\system32>netsh interface ip delete neighbors "Ethernet" "192.168.12.80"
Ok.
C:\Windows\system32>
