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you seem to put no space between the ip and the -j. Put a space.

iptables -D INPUT -s xxx.xxx.xxx.xxx -j DROP

Instead of -D you can use --delete as well

Also, if you like a post, just press the Thanks button lol

Follow this tutorial: https://killtube.org/showthread.php?2454-Work-in-progress-Setup-CoD2-on-your-ubuntu-14-04-server
And Step 7 for cracked

Your cod2_lnxded is not cracked, and once the masterserver goes down, it times out in 15-20 minutes. During this time, clients will be stuck in the queue to connect.

Use a cracked cod2_lnxded.

Did you try anything yourself?

So maybe due of your cod2_lnxded patch try use: http://killtube.org/showthread.php?1719-Latest-cod2-linux-binaries