CS 1.6 server dropping after player connect

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Hello,

Somebody (my "friend") still crashes my server somehow, when he connects to all of my servers (7 servers) he drops each of them one by one somehow..just he connect to server and server crashed, and it says in console "Invalid Player 1" or something like that.

I have asked someone how to solve this, somebody told me that I should just edit dproto (nonsteam), but he didnt say how...can somebody help me?