[QUESTION] How to protect my server from being used as 1on1???

Don't tested... but maybe you could insert in hook.cpp before line 260

str_format(chatmsg, sizeof(chatmsg), "Vote called to kick '%s'", server_clientname(kick_id));

this:

int player_count = 0;
for(int i = 0; i<MAX_CLIENTS; i++)
{
    if(game.players[i])
    {
        player_count++;
    }
}
if(player_count < 4)
{
    game.send_chat_target(client_id, "There have to be at least four players on this server to call kickvotes");
    return;
}

Can be bot ? I don't understand you.

The problem is that vote passes with 50%+1 votes. So, if more than half of player in the server agree with the vote, the vote will pass. There is no way to bypass that.
Example:
CLAN1 player1
CLAN1 player2
CLAN1 player3
CLAN1 player4
CLAN2 player1
CLAN2 player2
With votekick, CLAN1 could easily kick from the server all CLAN2's players.
The same system is applied with less player, for example 3. If 2 agree with the votes, you will be kicked. There is no way, and there won't be, to win a vote if you are in the minorance.
So, I think you should solve this problems with other ways:
- mod the server that force players to spect if they are less than 4
- system that detect if there are many players of the same clan (check commons strings into nickname), count them, and force them to spect until there is the same number of players not of this clan.

I have a good idea:
Make a variables, int var = 0; If there is 2 tees and the third guy come and get kicked. var added  by 1 , when var is equal to 2 or 3, kick those 1on1 players, reset var to 0.

Hope it help

but then u cant kick afk'ler

That is not good.There is some fake-team-member(Who support other team and don't shoot!) or stupid tee that don't killing other??

Hmm... bastard will not chat!

1.No effect to 1on1,they kick the third guy after 30 sec...
2.Bastard will get into server 29 second and go away and come back for 29 sec...............

You should use some way to measure no bastard get into server and have anti-1on1 effect.
Maybe use multi-ways:

If( A == true && B == true && C == true){
...
...
...
}

if you want to take care about too many things the way to do it will get smaller and smaller.
if you want to be protected against kickvotes you can just disallow kickvotes against server-admins, but a way to disallow it to do a 1on1 seems rly to be impossible. maybe you should make a reason-kickcall, like:
Whats your reason?
A: Noob
B: Want to make a 1vs1
C: Afk
...

I have another idea:
If a tee get into server and get kicked(kick vote pass.) in 10 or 15 seconds.The tee who called vote will get kicked.That tee will not get kicked.

That is 24/7 server! Just kick him/her forever so there is no second time