[Schneider]

pawn Код:

public OnPlayerTakeDamage(playerid, issuerid, Float: amount, weaponid, bodypart)
{
    if(issuerid != INVALID_PLAYER_ID)
    {
        if(weaponid == 0) 
        {
            new level = GetPlayerWantedLevel(issuerid);
            SetPlayerWantedLevel(issuerid, (level+1))
        }
    }
    return 1;
}

Edit: I wasn't aware of the existence of OnPlayerGiveDamage, so Dziugsas's code is easier, but add the weaponid-check from my code to see if the player used his fists.