If your furnace is an older model, you possibly have a combination fan/limit control with adjustable fan on/off settings based on temperature inside the heat exchanger. If the off setting is set too low, the fan will obviously circulate until that temperature is reached. If this is the case, hold the round dial so it doesn't move and slide the off pin to 100 degrees.
A newer furnace with a circuit board will have the same logic; shutting off the fan at a certain point, usually based on a certain number of seconds after the burner shuts down. Look for your control on the wiring diagram to see if it is manually adjustable. A temperature probe placed inside the heat exchanger helps to determine the actual shut down temperature versus the temperature selected on the control.
An even newer furnace will have an integrated circuit board in which the fan off setting is manipulated by dip switches. However, the maximum is usually not more than 120 seconds to avoid blowing air below body temperature. Integrated means beyond the dip switch selection there is no repair possible; the IFC would have to be changed. Again, do the temperature test to ensure you're not part changing for an incorrect reason.