1) Get access to the webGUI (after removing conflicting computer)
2) Go back to the mac address box in the webGUI and blank it out.
3) Click Save at bottom of page
4) I had to reboot and the original mac address came back.
Yes, this is very easy to do with pfSense. You will want a firewall rule on the WAN interface which allows access to the port you're running your webGUI from the source IP in question. Then you'll want to create a firewall rule on the LAN side (above the default allow rule) which blocks access to that same port from any source IP.
1. contact the person who was managing it before you
2. run nmap from a local machine using the command: nmap -p- -A 192.168.10.254
this may take some time, it will scan all tcp ports and try to connect to them to see what is running, look for a port that responds with:
"PortNumber/tcp open ssl/http lighttpd"