Please insert some images to describe what you want to do. 

You can apply a zero displacement BC at one location for the first time step, then you can Deactivate that BC for the second time step.

The problem is the other location, the nodes will have moved from where they were at the beginning. When you say you want to constrain those nodes, do you mean at their current location (which makes sense) or do you mean at their original location (which makes less sense)?