I recently tried to discourage robins from building a nest on top of an exterior light fixture near my front door. I can’t blame them, really, it’s an ideal location: under the corner of a wide eave, and on top of a solid and stable light fixture. What worked for me there was to place an inverted plastic soup container over the top of the fixture. Looks-wise, it doesn’t detract much, because the milky grayish texture of the container didn’t stand out from the siding color much, but it apparently made for an unstable and slippery place to establish a nest.