I once dirtied a window with a dark-colored spray paint sufficiently so
that the glass's reflectivity was dulled enough to make the bird not think
another robin was present in its territory. If you don't mind living with
slightly muddy-looking windows it might work for you. I only had to do
this to one window for one breeding season; the bird ignored the upstairs
window right above it. In the (dare I say it?) Pacific Northwest where
the breeding season is relatively short, a sullied window wasn't too bad,
but in an area where the bird is present year around, I don't know. Hope
this helps.

Charles Easterberg
easterbg at u.washington.edu