[JimmyCh]

Not sure if this is what you're asking for, but try the following:

pawn Код:

public OnPlayerTakeDamage(playerid, issuerid, Float: amount, weaponid)
{
    new Float:HP;
    GetPlayerHealth(playerid, HP);
    if(weaponid == [ID of weapon you want no damage on]) SetPlayerHealth(playerid, HP);
    return 1;
}

This will do no damage with a specific weapon ID, you can add more weapon IDs with the same format.
Hope this is what you're asking for.