[M4S7ERMIND]

Search for OnPlayerHealthChange callback. You can find it in YSI (YSI\System\YSI_callbacks.own)

Code:

SetDisabledWeapons
Warning: This function is removed in SA-MP 0.3. Alternatively you can return 0 
in OnPlayerUpdate if they use a bad weapon, and change their weapon

if thats what you mean be 'set damage of X weapon'.