Ha ha. Example----your computer you want to control has the ip address of 192.168.1.2, and the one you want to access is 192.168.1.3. The router is attached via fastethernet 0/0 to a switch, to which the two computers are attached. You cannot deny all inbound traffic to 192.168.1.2, because then it can't build an arp cache. IP traffic, yes. So, you need an extended acl.
router>en
router#conf t
router(config)#access-list 101 permit ip 192.168.1.2 0.0.0.255 192.168.1.3 0.0.0.255
router(config)#int fa0/0
router(config-if)#ip access-group 101 out
router(config-if)#^z(which is control-z)
router#
That should work. As soon as you make an acl (the first statement is always permit), then it inspects all traffic on the interface to which you apply the acl, in the direction you specify. With this one statement acl, it denies everything else, aside from of course the traffic it permits, which is ip traffic coming from computer a and going to computer b. If it does not work, post a reply again. Sorry for being a smart ass, but it seems if you know nothing about that which you are asking, how the f*** can you understand the answer? Something tells me you are not in charge of configuring the devices at this place, and if you are, they are in serious trouble...