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You have two alternatives here.

Send the message to everyone EXCEPT the player joining:

pawn Код:

new name[MAX_PLAYER_NAME], string[85];
    GetPlayerName(playerid, name, sizeof(name));
    format(string, sizeof(string), "{FF0000}[SERVER]{FFFFFF}: {CCCCCC}%s {FFFFFF}has joined LSCNR", name);
    for(new i = 0; i < MAX_PLAYERS; i++) //I would recommend using foreach instead.
    {
        if(!IsPlayerConnected(i)) continue; //Remove this if you use foreach.
        if(i == playerid) continue;
        SendClientMessage(i, 0xFFFFFFF, string);
    }
    SendClientMessage(playerid, -1, "Welcome to LSCNR {0066CC}cops {FFFFFF}and {FF0000}robbers");
    SendClientMessage(playerid, -1, "-------------------------------------------------------------");
    SendClientMessage(playerid, -1, "Please read {FF0000}(/rules, /help, /commands) {FFFFFF}before you play");

or

Send extra lines to the player to 'hide' the join message:

pawn Код:

new name[MAX_PLAYER_NAME], string[85];
    GetPlayerName(playerid, name, sizeof(name));
    format(string, sizeof(string), "{FF0000}[SERVER]{FFFFFF}: {CCCCCC}%s {FFFFFF}has joined LSCNR", name);
    SendClientMessageToAll(0xFFFFFFFF, string);
    for(new i = 0; i < 100; i++) SendClientMessage(playerid, -1, " ");
    SendClientMessage(playerid, -1, "Welcome to LSCNR {0066CC}cops {FFFFFF}and {FF0000}robbers");
    SendClientMessage(playerid, -1, "-------------------------------------------------------------");
    SendClientMessage(playerid, -1, "Please read {FF0000}(/rules, /help, /commands) {FFFFFF}before you play");

Note that it is actually more efficient to add 100 lines of 'SendClientMessage' rather than using it in a loop. But who wants to do that, right?

Those are your two options, choose your preference.