how to stop a belt?

Hi all
I want to stop a belt conveyor. I tried to unfix a "mesh/surface" and then fix it with a new surface_vel, but an error message shows up.

here is my in file:

########################################
fix belt_up all mesh/surface file belt_up_min.stl type 3 scale 0.001 rotate axis 0 -1 0 angle 0 surface_vel 0 0 0
fix inface all mesh/surface/planar file ins_min.stl type 2 scale 0.001 rotate axis 0 -1 0 angle 0
fix chute_a all mesh/surface/stress file chute_a_0.stl type 2 scale 0.001 wear finnie
fix chute_b all mesh/surface/stress file chute_b_0.stl type 2 scale 0.001 wear finnie
fix faldon all mesh/surface/stress file faldonmin_0x0.stl type 2 scale 0.001 wear finnie
fix belt_low all mesh/surface file belt_low_min_0x0.stl type 3 scale 0.001 surface_vel 0 0 0

fix granwalls all wall/gran/hertz/history mesh n_meshes 5 meshes belt_up chute_a chute_b faldon belt_low

#distributions for insertion
fix pts1 all particletemplate/sphere 1 atom_type 1 density constant 2500 radius constant 0.010
fix pts2 all particletemplate/sphere 1 atom_type 1 density constant 2500 radius constant 0.0125
fix pts3 all particletemplate/sphere 1 atom_type 1 density constant 2500 radius constant 0.014
fix pts4 all particletemplate/sphere 1 atom_type 1 density constant 2500 radius constant 0.015
fix pts5 all particletemplate/sphere 1 atom_type 1 density constant 2500 radius constant 0.016
fix pts6 all particletemplate/sphere 1 atom_type 1 density constant 2500 radius constant 0.017
fix pdd1 all particledistribution/discrete 1. 6 pts1 0.2 pts2 0.2 pts3 0.2 pts4 0.2 pts5 0.1 pts6 0.1

#region and insertion
group nve_group region reg

#particle insertion
fix ins nve_group insert/stream seed 1000 distributiontemplate pdd1 &
maxattempt 100 nparticles 10000000 massrate 167 overlapcheck yes vel constant 2.2 0.0 0 all_in yes &
insertion_face inface extrude_length 0.5

#apply nve integration to all particles that are inserted as single particles
fix integr all nve/sphere

#output settings, include total thermal energy
compute 1 all erotate/sphere
thermo_style custom step atoms ke c_1 vol cpu
thermo 100
thermo_modify lost ignore norm no
compute_modify thermo_temp dynamic yes

#insert the first particles so that dump is not empty
run 1
run 200
unfix belt_up
#fix belt_up2 all mesh/surface/stress file chute_a_0.stl type 2 scale 0.001 wear finnie
fix belt_up2 all mesh/surface file belt_up_min.stl type 3 scale 0.001 rotate axis 0 -1 0 angle 0 surface_vel 0 0 0
run 1000
########################################

can anyone give me some guidelines?

Best