You cannot remove permissions from a user already granted, therefore, by providing read/write access at the root of the repository to the
@devgroup you have provided it to all sub-trees.
The way I would achieve this is by segregating your project areas into different repositories with different permissions and then using the
svn:externals property to bundle these into the top level repo.
svn:externals property can be set on any versioned directory, and its value is a multi-line table of subdirectories (relative to the versioned directory on which the property is set), optional revision flags, and fully qualified, absolute Subversion repository URLs.
$ svn propget svn:externals .
When someone checks out a working copy of the project repo , Subversion also continues to check out the items found in its externals definition.
$ svn checkout http://svn.example.com/repos/project
Checked out revision 148.
Fetching external item into 1
Checked out revision 14.
Fetching external item into 2
You can then place restrictions upon the two restricted repositories to prevent the
@devgroup users from updating these, whilst still getting them when they checkout the main project repo.