You can use a router as a form of Firewall but you need to know a lot of things in advance. If you just want to stop a certain type of packet coming from a known IP address, yes you can construct and ACL to prevent this - but as has been mentioned in the above post - it is not terribley hard to circumvent an ACL.

ACL's come in handy as "internal firewalls" to filter out certain types of packet etc on your network but if you are lookjing for something to put out as your border defense for the entire network - get a firewall!

An example of an ACL is:

Code:
access-list 101 deny tcp any any eq 31337
This will deny TCP traffic from any host to any host directed towards port 31337.

This is just one rule - but can you see the problem with using ACL's instead of a fire wall? All this rule has done in prevent TCP traffic destined for one port - 31337.

So you now have one port secure!
Only another 65535 acl's to write and you can have all your ports filtered!