Realize that the other person is free to ask anything they want, but just the act of them asking doesn't create any obligation on your part. Especially with people where you don't have a longstanding relationship, you don't owe anyone anything more than a polite no.
The other piece I learned was this useful phrase to deploy when under severe pressure to give a reason and for whatever reason you can't just disengage or leave: "Sorry, I'm just not comfortable with that". No one can reasonably argue with that, and if they do, having already given your reason you can then simply fall back into a loop of "No, sorry."