Iptables: howto reject syn-scan and the others

Hi Lists, I have set up my own iptables rules, when I portscan myself with nmap -sT I see all my ports closed. When I try to portscan myself with nmap -sS or other scan types I see my ports "filtered" I would like to see them "closed", but it looks like I need to reject those packets not only with icmp-port-unreachable but with tcp packets (reading nmap man page). How can I reject packets with tcp packets as reply? Praise