Cut caution tape or shiny polyester film into strips a foot or two long. Cut six to eight strips, or more if the bird-friendly area is rather large.
Thumbtack one end of each caution tape or polyester film strip beneath the overhang so that the strips hang down onto the flat area that potentially attracts nest builders. Tack the material in such a way it can move with the breeze. Cover the entire nesting area with hanging strips, making the area less attractive to birds.
Cover the potential nesting area with plastic netting. Cover the entire area so that birds cannot land on any ledges or get behind the netting.
Place an angled wood block atop any ledges where netting isn't an option. A steep angle covering all the previously flat areas will make those areas less attractive to nest-building birds.