I've heard from other members before that if you press forward and the sprint button at the exact same time you'll skip the punch, but what I usually do is repeatedly set the player AI and that just seems to work for some reason. Generally you can just do...
PedSetActionTree(gPlayer,"/Global/G_Striker_A","Act/Anim/G_Striker_A.act")
while true do
PedSetAITree(gPlayer,"/Global/PlayerAI","Act/PlayerAI.act")
Wait(0)
end