Why not use the out of the box tagging features Kentico has vs. building your own? You could create your own form control then that does some custom work to display the tags for a specific group so there is no free form entry.
If tags won't work (seems like a bit more coding that needs to be), I'm pretty sure you can use categories and create a category filter for those items on the front side. This will allow exactly what you're looking for.