Skip/freeze particles for saving computational time

I'm setup a simulation where I'm pouring particles into a geometry to form a stack of particles. The particles are the agglomerating together with a cohesion model and I notice that the computational resources is increasing with increased number of particles (as one would expect).

For many of these particles they will not move anymore since they are just agglomerated together and have many contacts so I would like to skip the calculation on these particles and just freeze them instead such that Liggghts does not calculate the contact forces and they will then just act as a wall.

I would like to do this based on the coordination number of the particles but I don't now how to do this, or if this is possible? I was thinking to use the "freeze" command, does this do what I want to achieve with saving computational resources?

What I was thinking of doing is something like this: (But this doesn't work apparently the command for the group definition is wrong)
compute coord_nr all coord/atom ${diameter}
group freeze_group c_coord_nr >= 10
fix freeze_fix freeze_group freeze

Best regards
Tobias