Force required to stop an object initially moving with constant velocity

[AdvaitDhingra]

Let's assume I have a ball moving at a constant velocity and it collides with a spring and the spring compresses n cm. If I know how much mass the ball has and the spring constant D, how would I calculate the Force? I mean since F = dp/dt I would have to know the time in which the stopping occured. How can I solve this?

 

[ergospherical]

Do you want the maximum force, the average force (time-averaged, or distance-averaged), or the function $F(t)$?