Sorry in advance if this has already been posted, but I only encountered this today. I'm on PS4 and had both a kidnapping and ghoul-cleansing quest from Preston.
I was tired of his quests, but did do the objectives up until going back to Preston to talk to him. So I saved the settler and killed all ghouls in another location.
Then I just left the quests open. Turns out they eventually "complete" themselves I had no idea about this, it's gonna be a long while before I'll get back to Preston - if ever...!!