What you should be doing is just letting the birds out once a day for an extended fly and then call them in for food. I used to do this when I had racers in the evenings, just before their supper.
You can get Danzig Highflyers and they will pull your flock up so high you won't be able to even see them. Then you can either do a call and/or throw a white fantail on the roof as the signal to come in and they will all descend at once.
This will keep the wild ones out as they don't want to fly so high. You can get disease problems introduced from any birds that have come in from the outside, even errant racing birds, so you should either get rid of them immediately or quarantine them.