How to kill all active tcp connections from an IP?
I am using CentOS
If you're running iptables and assuming you're talking about tcp traffic:
iptables -t filter -I INPUT 1 -p tcp -m tcp -s <ipaddress> -j REJECT
Should work. Probably don't need "-m tcp" but won't hurt.
Thank you for your quick reply. 
If this is some sort of attach then you want to change '-j REJECT' to '-j DROP' No use in responding to them and wasting cycles.