House sparrow vs. bluebird help

A persistent pair of house sparrows has been attempting to nest in a bluebird house despite my (and the bluebird's) best efforts at keeping them away. Does anyone have advice? I tried stringing fishing line parallel to the opening of the box but that didn't keep them out. The sparrows even nested elsewhere in some sculptural tree roots we were saving (see photo), but they seem to insist on claiming the bluebird housing. What's worked for you?

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