how do i make BodyForce slow down when the force is lowered? or come to a stop when it is set to 0?

****THIS IS BEING EXPLAINED USING A 0 GRAVITY ENVIRONMENT****

I'm making a space related game and im having trouble with making the ship mobility system. I'm trying to create a system using body-force that will speed up the ship to a certain speed depending on the speed input by the player. then when the player slows down or wants to stop i want the ship to stop. However since there is no opposing forces in space its hard to figure out how to slow down the ship to the speed desired by said player. Any help?