Draytek Vigor 2672.
Can someone please explain in detail, but in the simplest possible terms, how to block a specific computer from accessing the internet, but allowing access to the Lan.
The computer's ip addess is fixed by binding it to it's MAC address, and is 192.168.0.110

Easy: Use the Firewall.

In the Firewall menu, under the Data Filter section, create a new rule. 

Direction: LAN -> WAN.
Source: 192.168.0.110 (Computer's IP).
Destination: Any.
Service Type: Any. 
Block Immediately.

Make sure the rule is enabled, and that should block all outgoing connections from the computer to the internet, whilst still allowing it to access not only it's own LAN subnet, but any others you may have configured. (As long as the appropriate routing settings are in place).

See how that works.