Keeping structures at fixed position

Dear lammps user
I am trying to model a cnt-polymer nanocomposite with reaxff. But As I start lammps simulation, cnt atoms get dispersed and loose structural integrity. How can I keep the cnt fixed within a certain region so that atoms don’t get dispersed during relaxation. Which command should I apply. I tried with fix setforce command with 0 0 0 option but didnt work.