I am trying to find a way to block a certain Mac address / Internal IP from accessing the internet (Blocking a device in the LAN to WAN) in python.
This option is available in every modern router in every home but mine is kinda old and doesn't have that feature.
I have a basic knowledge in networking stuff and consider myself an Advanced-Beginner in python, so I'm up for the challenge but still need your help.
*Of course with the option to enable the internet again for that device
I know I am kinda late now but... You can't necessarily block internet access to a machine like you would do in your router's config.
What you CAN do is implement something like an ARP Spoofer. Basically what you would do in a Man-in-the-Middle attack.
You send a malicious ARP packet to poison the target's ARP table. Making it believe your machine is the router/default gateway. That way you can intercept every packet being transmitted by the target. You can then choose if you want to router them or not.
If you choose not to forward the packets, the connection to the internet is cut off.
If you want to forward the packets to the actual router (in order to allow the target to access the internet) you must enable IP Forwarding on your machine.
You can do this by running echo 1 >> /proc/sys/net/ipv4/ip_forward on Linux or changing the Registry Key in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\IPEnableRouter on Windows ('1' forwards the packets, '0' doesn't). By default IP forwarding is set to 0.
Remember you must resend the malicious ARP packet every couple of seconds as the ARP tables get updated quite frequently. This means you don't necessarily have to change the IP Forwarding configuration on your machine. After a minute or less of exiting the script the target's ARP table will go back to normal, giving them access to the internet again.
Here are some python modules you might want to take a look at:

Scapy (Packet Manipulation Tool)
winreg (Windows Registry)
How can I block Internet access for a certain IP in my local network in python?