I tryied many solutions found on the forums. Finally, after many tests I Found the solution who work:
# iptables -A OUTPUT -p TCP -d 0/0 -dport 5050 -j REJECT
This will block all tcp packets to [ANY IP] having destination port 5050. Simple and eficient … works well on my Fedora Core 6 Internet Gateway. Yahoo uses many servers for backup, and it’s dificult to find all, but all uses same port to connect … 5050 it’s default port for messaging/chat.