The springs in MDO are idealizations of the real world, so they are limited to always be "connected" to two bodies or act directly on a joint axis. If you wish to model the behavior you describe, you can do it. One way is to model a third body that has a "liftoff" cam pair to the body that the spring is compressed by. This way, after the constraint is removed from the body connected to the spring, the 3rd body will accelerate and loose contact with the body attached to the spring.
The 2nd way is to model the spring as multiple bodies and have a liftoff cam pair created between the top coil and the body you would like to "launch". This method would allow you to capture the non-linear spring dynamics when they exist for your system. I did this countless times many years ago for engine valve springs with very accurate results for high-speed dynamics.
Good Luck!
