Your easiest solution is training. Put the coordinate systems you want them to use into Favorites, or have a network share where they "import" from that location.
A drastic solution is to make use of the projection engine's 'back door', an environment variable. Through it, you could turn off the built-in database (collection of predefined coordinate systems), then add in only a selection of what's there. It can cause issues with other parts of the software though that are looking for particular coordinate systems (if you don't include them in your custom list). Connected to that, is that you don't turn off the built-in database, but add in a new "folder" to the coordinate system picker where the folder is your company or locality name.
Another way would be to code up something. I don't think you could block using the 'create new feature class' tool, but could have a custom tool that doesn't the same thing, but with the coordinate system preset. I don't have enough coding skill to even be able to tell you how to start doing that.
Melita
