[Quickie]

pawn Code:

new AntiAFK[MAX_PLAYERS];
new AFKTIMER[MAX_PLAYERS];

#define MAX_AFK_TIME 5

forward OnPlayerAFKcheck(playerid)
{
    AntiAFK[playerid]++;
    if(AntiAFK[playerid]>MAX_AFK_TIME)
    {
        // ur code
        //SetPlayerHealth(playerid,0.0);
    }
}

public OnPlayerConnect(playerid)
{
    AFKTIMER[playerid]=SetTimerEx("OnPlayerAFKcheck",1000,true, "d", playerid);
    return 1;
}
public OnPlayerDisconnect(playerid,reason)
{
    KillTimer(AFKTIMER[playerid]);
    AntiAFK[playerid]=0;
    return 1;
}
public OnPlayerUpdate(playerid)
{
    AntiAFK[playerid]=0;
    return 1;
}