First of all, this is actually impossible. Unless YOU are the alcoholic, you can't stop an alcoholic from drinking. Alcoholism is its own demon that can't be fought from the outside. What you can do is make it impossible for the alcoholic in your life to drink without having the natural negative consequences that come from drinking. You will have to practice stepping away and watching the alcoholic face (or not face) his responsibilities. It will seem alien, harsh, and like you aren't helping, since this "stepping away" seems contrary to "helping." And the alcoholic will surely let you know that it feels this way. But be confident. It's the only way. Here are some basic ideas to help get you started.
How to Help an Alcoholic Stop Drinking
Get yourself out of the house and to an Alanon meeting. You'll find a lot of people just like yourself who are trying to detach from an alcoholic situation, leave the alcoholic alone, and get some control over their own lives.You might find that if you go to Alanon meetings regularly, your worry over the alcoholic will lessen. You'll start to realize that the alcoholic's problems belong to the alcoholic, not you. Watch out -- you might start feeling great.
Do not give the alcoholic money. Do not give him your car. Do not let him live in your house without paying rent. Do not lie to his boss to save his job. Do not lie to his wife to save his marriage. Tell the truth about everything. It will be hard to do, but it will help the alcoholic face the consequences of his drinking.
Take yourself out of the alcoholic's life. This isn't to punish the alcoholic -- this is to save yourself. Let him know you love him -- but that you can't watch someone kill themselves with alcohol. Tell him to call you when he's sober and in a program.
Focus on your own life. Plant a garden. Have a baby. Get married. Go on vacations. Enjoy your life. You can't control anyone's behavior but your own, and you deserve a life you can enjoy.
Trust yourself. You are doing the right thing. If your loved one was sober, and you were the one suffering with alcoholism, you would hope that they would love you enough to tell you the truth, and give you the opportunity and the dignity of fighting your own battles.