No don't fill it in with anything, you need to find out the course of the overflow and rectify the problem. Basically the water still has to go somewhere! Do you find the water overflows when it has been raining heavily? Is your well connected to the water table? etc etc. I would find out as much info as you can before contemplating any work. Try and obtain information through various sources, sometimes the old timer who lives in your street might know something about where and for what purpose the well was constructed.
Hope this helps