For each moving object you have an associated 6DOF properties UDF in which you set e.g. the mass and the moments of inertia:

  prop[SDOF_MASS] = 907.185;
 prop[SDOF_IXX] = 27.116;
 prop[SDOF_IYY] = 488.094;
 prop[SDOF_IZZ] = 488.094; 

and other 6DOF properties. The entries of the 6DOF properties array are explained in the section on DEFINE_SDOF_PROPERTIES in the Fluent Customization Manual.

To constrain the motion of a 6DOF object, i.e. to set the motion to 0, you simply set in the UDF the corresponding 6DOF property array entries to TRUE, e.g.

  prop[SDOF_ZERO_TRANS_X] = TRUE;
 prop[SDOF_ZERO_ROT_Y] = TRUE; 

where SDOF_ZERO_TRANS_X, SDOF_ZERO_TRANS_Y, and SDOF_ZERO_TRANS_Z are the components of the translation and SDOF_ZERO_ROT_Y, SDOF_ZERO_ROT_Y, and SDOF_ZERO_ROT_Z are the components of the rotation.

By default these entries are FALSE, i.e. the motion is not constrained.