My Son Is Stealing Money From My Purse. What Should I Do?

This is obviously a distressing situation to be in.  However, there is some underlying reason why he feels the need to steal from you.  Is he being bullied and therefore needs the money to pay his tormentors?  Is he very materialistic and needs money to buy flash clothes or technical gadgets?  Is he making a statement to you ?   Is he taking drugs?
Some psychologists think that when someone steals money from a family member they are actually trying to get love.  Money and love apparently have deep rooted psychological alliances.  So at the heart of this, your son may not feel truly loved and so he steals from you.
Try very gently talking to your son and explain that he is loved and cherished and if anything was wrong in his life he needs to tell you and then together you can sort it out.  
Communication needs to be honest and open.  Don't just hide the purse and hope the problem will go away, since if he is so desperate for money he may turn to crime.  Instead, get into a dialogue and get to the cause of this problem, not just the symptoms.

You don't say how old he is.

Age is an important factor in the way to respond.

Beat his ass.

First of all sit down with your son and talk, why he needs money. Provide him whatever amount he needs and keep a check on his activities, i.e what he is doing with it

I am sorry you are having this problem. No one wants to think their child is a thief. But whatever the problem, let him know you are aware and this behavior will not go on any longer. First your purse, then your jewelry, tv's, computers car etc. And when all your things are gone , its the people in the neighborhood that will be the next victims. He needs help & prayer. He has your love, but now he needs your strength. If he is a minor, get him some counseling, if he is an adult offer your support and try to get him to seek help.