How can I disable any external traffic to my port 80?
You'll want to be sure you have an allow rule with a lower number than the deny rule.

So perhaps:

00100 allow ip from any to any via lo0
90100 deny tcp from any to any dst-port 80 in


This puts the allow very low (make sure you don't have any deny rules in the 0-99 rule number range) and puts the general deny rule at a very high number (presumably you won't be allowing anything on port 80 higher than 90101).

If you are not defining a rule file to load all the commands in one shot, have a look at this answer for a slightly different take on restricting ports: How do I use ipfw to allow LAN access but deny Internet access?