If the geometry should be fixed in Z - direction, go to

Static Structural > Insert > Displacement.

Now select the corresponding geometry and assign the Z component of the displacement to be a constant value (here 0 mm).

Putting boundary condition in z direction as follows

load-displacement- z direction- value 0( if no movement ),

if you know how much structure will move in z-direction then put that known value.

Thank you,Mr Chitaranjan Pany & Mr Naveen Krishna Baru

In addition to all these correct answer, it is possible to define a function for the boundary condition as well in case the boundary condition needs to follow an explicit function.

If possible, you may introduce a frictionless support perpendicular to the z-axis.