Is there a way to stop some to scan my computer for opened ports? Someone i know did something so i taks about a min to scan (the one who scans gets bored)
Any ideeas?

Did you try disabling icmp echo reply? I've noticed it takes longer when scanning machines which don't reply to nmap's pings.

I figure, if your firewall rules are strict enough your desired result may be acheived.

For example, setting your INPUT policy to DROP and adding other rules to allow what you want may help.

Look into firewall rules.

the usual (read : nmap) scanners usually use SYN tcp flag to know if a port is open or not. So limiting the number of syn flag you allow would really slow down a scanning ( it could maybe even make it hang I believe). So setting a limit rules about SYN flag would be an idea.
HOWEVER, be aware this might be used against you, someone knowing the tricks could just send you endless stream of SYN to make your computer stop responding to connection (basic DDOS). If you plan to use this, make sure to place this carefully in your firewall, make sure any critical service ACCEPT communication before the limit rule is checked.

Ps : if you just want port to appears as CLOSED instead of firewalled, you could use "-j REJECT --reject-with tcp-reset" instead of DROP. This often confuse ports scanners.

try portsentry the best about that stuff AFAIK also manageable via webmin

if you change you iptables default policy to drop it will take forever to scan you completely, although it does'nt stop it. And don't blacklist people for scanning you because the source address can be spoofed.