I want to prevent an IP from accessing the internet / wan and am wondering
which method uses less resources and will thus be the quickest too.
Reject or Drop, which one, thanks
With REJECT the lan client will be notified a connection is not possible.
With DROP the connection is just refused, your lan client will keep trying but eventualy times out.
Usually for the wan side you use DROP, for the lan side REJECT
But it is up to you
As far as I remember default firewall rules uses only REJECT. Am I correct?
Yes I think so, I changed it to DROP for the WAN side 
Thanks for posting, REJECT will suit me best then, I read since I posted on a few
others sites and they advised the same, DROP if dealing with traffic coming in from the WAN and REJECT when dealing with trafic from the LAN side.
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Thanks!
Can you share your firewall config so others can replicate it without asking same question again and again?
I added this rule to keep a camera from accessing the internet / updating the mothership.
The image below only shows the first rule in the list, it is quite long.
There is an add button at the bottom of the list, use it to add new rules.
Navigate to Network > Firewall, then to Traffic Rules as shown at the top of the image below.
