Most likely the float valve isn't shutting the flow of gas off. If this just started happening after a period of non-use then you might try shaking the bike back and forth a bit or tapping on the carburetor to get the float to unstick.
If the problem has been developing and getting progressively worse then the needle valve and/or seat is probably damaged and will need to be replaced. I recommend buying a new needle, seat and gasket set for the carburetor and doing a complete cleaning. When you reassemble make sure that you adjust the float to the proper level (essential whenver you replace the needle and seat, the float height will change). If you don't already have a manual for your bike you will need to get one in order to get the information on the proper float height.