(1) Inform them in a private message, email, or chat that they are notifying you too much and they need not notify people of every single post that they create. Be polite but stern and clear with them.
(2) You can change who can notify you in the Google Plus settings. You can have only specific circles or allow all your circles to be able to notify you or the Public or you can customize it to include only specific people and circles. It's up to you. You have those settings. Just remember when you allow only a small amount of people (specific circles) to have the option to notify you that you will not get notifications about a private message or a great hangout from people. I would advise you to find another way that people can contact you if you want contact...
private email message enabled on your profile
Your link to your website
Your Facebook profile
Blog or another social media network
(3) If they continue to notify you too much, use the IGNORE feature.
You can access it in your drop down notification panel from the sandbar or you can go to your CIRCLES Page and click on Have you in circles tab and then you find all the people that notify you too much and select them and then click on Actions tab and then click IGNORE option. You will not receive any notifications at all from them: no direct post shares, no private messages....no mentions......NOTHING.
(4) You can just Block people.
The Notify feature is a great feature if used in moderation with consideration of your followers that you have in your circles. Do not abuse the feature.
Use it sparingly to bring attention to a great post or Hangout.
Make a circle that is just for notifying people. Allow them to OPT-In to the circle if they want to receive notifications from you. Make a post and ask your followers if they would like to get notifications from you sparingly of your most amazing posts.
Don't abuse the feature.
This isn't difficult. It's simple. Be an adult and deal with the notify feature as an adult that cares about other people. Be kind.