I would stop them visiting and actually have done in the past when certain visitors (family members, as in your case) persistently encouraged my dog to jump over the baby-gate and jump up at them for a fuss. He's a really friendly dog, so slightly different, but they were constantly reinforcing the opposite of what we wanted him to learn and consistently undermining all the training we were doing with him.
If they are people you don't feel you can stop visiting the house for whatever reason, I would remove the dog rather than exposing it to their behaviour.
Whenever you have visitors to the house that you feel, for whatever reason, aren't willing/able to follow your guidance regarding the dog, it's better to remove the dog from the situation than risk undermining her training and setting her back. So eg, when the guy comes to read the meter, it's probably easier to put her in the garden than expect him to participate in her training. 
(Although we have a plumber who loves dogs and is always keen to help. 
)
With people who are more willing to listen, work with you and help, the treat throwing thing will help, as will people being calm and ignoring. The more people you can get to do this with her the better.
Are you taking her to dog training classes? That's something else that will help, as there will be experienced people who can be introduced to her properly and you may also be able to arrange some willing stooges, who will come round to your house with the sole purpose of helping you with the process.