Are your Toolbox components in a separate folder inside the PDM vault? Normally a folder called Library etc.
In the PDM Administration tool, open their Group, go to Folder Permissions, click on the Toolbox folder, and the only permissions they need are "Read file contents" and "Show working versions of files"
This way they will have read only access to those files (ie still be able to use them in assemblies, but not check out and edit them)
This is my favorite method to accomplish what you’re looking for!