After forming the whole unit cell with a specific space group, one of the atoms has to be removed, provided that the other atoms of similar type within the unit cell remain as they are. How can this be done in materials studio?
Well, Materials Studio is a commercial collection of different stand alone codes. If you are dealing with point defects, as it seems, you will have to generate either supercells, either clusters... Each of those methods have both advantages and disadvantages, study them. I use some free codes for doing that (QuantumEspresso, Abinit) or licensed CRYSTAL09 - but, strictly regarding your question - try to use the code CASTEP, included in Materials Studio.
I think you mean that you want to remove 1 atom without removing symmetry copies, right? You need to convert the cell to P1 symmetry. This option is somewhere under the symmetry options, but I can't remember precisely where
Professor George Fitzgerald just gave you another good option to think on it. To break symmetry through just "declaring" the lowest one. Sure, the method will kill the computation time and much more, if you study periodic systems...
On converting to the P1 symmetry we can remove or add atoms, its true, but the unit cell will lose its identity and also does it affect the properties like band gap and density of states etc.