In order to plot a geom on only one facet, the only faceting group in the data should be the one to which the geom should be applied. For example, I have attempted to recreate your plot with the iris data. I filtered the iris data to only include the species "setosa" for the geom_hline:
In order to have geom_hline not be global, you need to subset the data supplied to geom_hline to only the values of 'variable' which you want to plot the geom_hline as in my example. I only want geom_hline to apply to the 'setosa' facet of 'Species' so I subset the data provided to geom_hline to only include 'setosa'.