d_loayza
Yes you can achieve blocking of Internet access based on mac address.
You have to configure your system appropriately.
Please reread my previous answer which, slightly rewritten, says:
Add the workstation hostname (eg station15) to the Hostnames and Addresses panel in server manager, quoting the mac address of the workstation NIC, and the required IP eg 192.168.1.15.
This will force the SME servers DHCP server to always issue the same specified IP address to that workstation (that has that unchangeable mac address, assuming the NIC is not physically changed).
You should apply this to all workstations on your network.
Then, for any workstations that are allowed to have any access to the Internet, add these "auto allocated fixed" workstation IPs to the exceptioniplist file.
Normal Dansguardian filtering rules will still apply even to allowed IP's. You could make these DG rules more strict ie decrease the naughtyness score and add more restricted site lists, block many file types etc etc. Have a good read of the wiki article again and review all the configuration files as there are useful tips in the start of each file. You could even setup Filter Groups so you can be more strict with some users and less strict with other users basedf on logge din user name.
You need to take the approach in Dansguardian to "block everything", and only allow specific workstation IP's to have access.
Even if clever users can still change the workstation IP, this should prevent them gaining access as any "free" IP they use are not allowed access.
Furthermore, you should also ideally enable Group Policies using gpedit.msc (when logged in as Administrator on the workstation), or by using centrally administered security policies (poledit etc), and restrict the workstations users ability to make these fundamental changes to workstation network settings etc.