Another Apple computer in synch with my own?

I have a slow speed computer in synch with my own iBook 1.2 GHz Power PC G4 which is causing some performance problems. Also I notice this computer in synch with my own is causing some application problems with someone approving Options Trades that I would not approve myself. This is producing financial loss while I am having trouble controlling the decision making on the trading platform. The approve button automatically is pressed by someone else while I would not approve the transaction. I think this is an Apple Support problem.

Also when I check my firewall I notice the IP address 192.168.1.254:53 is attempting access on a daily basis. https://discussions.apple.com/thread/4047778?start=0&start=0 This user may be a problem with eye abuse and psychiatric nursing. I would prefer to keep the aggressive users from disrupting my work. Is there a way I can block this IP address from getting access?