Install yum-priorities and give dag a higher priority. This will make sure that nothing is pulled from it unless it is not available in the main repositories. You can use the "exclude=" setting on the base repositories if there is something there that you would rather get elsewhere.
Excellent - about to test now!
This also allows me to set my internal postgresql repo as a higher priority than CentOS so that I can avoid some of the other fancy footwork I'm doing!