When placing a bounty on someone, very often that character learns of the bounty and quickly pays it off without any chance for an assassin to do what they do. This is double troublesome for settlement leaders who have a full settlement's funding to pay off bounties.
The suggestion is to have a grace period after a bounty is placed where it can not be paid off right away. Perhaps the guild will confirm there is a bounty, but states a cooldown until they can have it paid off. I wouldn't know the appropriate amount of time for this grace period, so can only throw out having it be anywhere from 6 RL hours to a full day. Maybe it's longer if you're a settlement leader?