There are some new consultants, who shall be working on parts of a software which aren't functionally critical to the overall software. The software is not organized in a modular fashion, s.t. all of these source-files that hold the non-critical functionality, are not isolated in a directory.
Is there a way such that, the consultants are not able to fork/branch the project, and also can work only on files (the non critical functionality files) ?
The version-control software is SVN in this case.
If you use VisualSVN Server on Windows you can easily set access (eg read-only or no-access) permissions on the directories. You cannot set anything on a per-file basis in SVN (it was designed to operate on directories). Other SVN systems can do the same by editing the authz file and editing an entry for each directory you want protected.
Setting access controls on individual files will quickly become a nightmare, so move the files you want protecting into a couple of directories and put your controls on them.