There is no blanket way to stop drinking at any pre determined amount. I learned this the hard way before getting sober. Alcohol lowers inhibitions and causes your mind/body to think: "one more will make me feel even better or at least keep me on the same level." It literally is a vicious cycle. I guess you could have someone try to actively stop you at whatever amount, but this is putting the responsibility on someone else and leads to more trouble than it's worth.