Linda,
I hope you are getting lots of responses to your question about repelling house sparrows (bluebird people call them "HOSP") from your swallow nest boxes. We had the same problem here with our resident Western bluebirds ... HOSP showed up one year and the next year they were taking over nestboxes, and before we knew it, they killed some bluebird nestlings and the bluebirds never nested here again!
Since you mentioned the Internet, I am guessing you are researching your dliemma there, and that's a great source, but to put things in perspective, it's important to understand a few behavioral things the HOSP does that are actually brilliant adaptations:
1. They nest early, almost before anyone else, with males staking out nestboxes and calling their distinctive "come hither" call. I saw one doing it yesterday in the snow here! This is a good time to have nest box entrances blocked off! Our bluebirds come in pretty late, and one of these years I am confident that they'll come back. They still come check the nests.
2. Some of the bluebird people will trap and kill sparrows, but I will not. My yard is not a killing place. BUT, I will remove sparrow nest material from nestboxes (they pile in a lot of stuff!) as they put it in, and if I ever found eggs, I would remove them, too (but not chicks). I think vigilance is imperative, and some would say that if you can't monitor and control your nestboxes, you should take them down. I live on an acre and now only have one bluebird box and one flicker box, where before I had maybe 6-8. Bad me!
There is actually a huge problem right now, with well-intended people buying nestboxes - especially those cute little ones - and putting them up all over their property, and in unmonitored yards, they're mostly providing sites for house sparrows to flourish!
At the very least, if you can't entice the swallows to nest there again, don't let sparrows raise young on your property, and you can do that by removing the houses. If they figure out other places to nest, well, bless their hearts, but not at your hand (at least that's my philosophy).
3. House sparrows are ground seed eaters and as such, they're not as vision dependent as bluebirds or swallows who feed on the wing. So, someone came up with an idea to string monofilament (fishing line) around birdhouses, and you know, I think we've had success with that. The idea is that HOSP will come zooming toward a house and at the last minute they'll see the monofilament and it will freak them out. The other birds, already more visually adept, find it no problem at all, so they just whiz past it.
There are lots of references to this on the Internet, so I'd google "monofilament and house sparrows."
In the end, maybe you should remove the nest boxes and take the patch job off the vent hole screen where your swallows first nested and see if the swallows will return to that. I mean, what harm can they do in there anyway? Good luck and keep us posted and everyone ... be vigilant!
Good birding and good stewardship,
Linda Hiller in northern Nevada (former Washingtonian)