It has some way to prevent this?

Those missions are unrelated to your defenses, those are just repeatable radiant quests.

you can thank Preston for those. it becomes very repetitive after awhile.

I found that I can build a large 5x5 house by hand, completely scrap it and repeat. do this a couple times and get more XP than doing the radiant quests.

What's funny is that when I do get a rescue mission, it is invariably one of the Finch's sons. That boy just KEEPS getting into trouble!

First one of those missions I did, I actually escorted the kidnapped person back to their settlement. After that, I just fast-traveled back and the mission completed. Whether the hostage made it or not, I apparently don't care: I rescued them, I get credit.

There's no way of preventing them with defense. The only way to stop them is to complete the quests that are logged, and don't talk to Preston afterward, or just fail the current logged quests, and send Preston to a settlement that you don't go to often so you don't encounter him, don't listen to Freedom Radio, and turn off the speakers (with the blue breaker box at the bottom of the tower) at the Castle. They're infinitely repeatable radiant quests.

I never do the kidnapping quests myself. I just pay the 550 cap ransom. seems to work just fine.

That is , man thank you