could you just mark the shapefile read only in the operating system? It's not secure, but it will keep honest people honest - I.E. it won't accidentally get changed. You mentioned that this is an independent shapefile, but then you state you don't want the entire geodatabase locked down, so I'm not sure if you are meaning this is a feature class file in your geodatabase or a truly independent shapefile... but if it is the .shp, .shx, and .dbf set of files then you should be able to use the operating system to mark them read only.