How can I get my I phone to stop sharing iMessage with my Mac book?

Disable Forward SMS on your iPhone and sign out of your Apple ID in Messages on your Mac.

First, on your iPhone go to:

Settings → Messages → SMS Forwarding → disable your Mac.

Then on your Mac:

Open the Messages app. Select Settings up in the top menu, click iMessages and select Sign Out.

Now you will no longer receive messages on your Mac.

I don't see it as a forwarding issue. If I have this setting on and I send a message on my phone or receive a message on my phone, it will show up on my Mac since it will be synced with iCloud and I have the same Apple ID on both the Mac and Phone.

With this setting off, the messages will no longer show on the Mac. Of course in that same iMessage tab I referenced before you will want to be sure just below the "Enable Messages in iCloud" where it says "You can be reached at:", that your phone number is not one of the locations checked.

This way you can use Messages on both devices signed into the same Apple ID with different conversations and no sharing of messages.

On your MacBook:

1. Open Messages
2. Then Settings... under the Messages menu
3. Click the iMessage tab
4. Disable the "Enable Messages in iCloud" setting

This will not stop sharing between iPhone and MacBook, this will only stop Messages from syncing in iCloud as far as I can see?

Disabling Messages in iCloud will not prevent messages from being forwarded from iPhone to Mac. Neither iMessages nor normal Text Messages. It will only prevent the messages from syncing, so if you delete a message on your iPhone, it will also be deleted on your Mac. All messages will still be delivered unless SMS Forwarding is disabled on iPhone and iMessages is disabled on Mac. But you are absolutely right that if he deactivates his number in "You can be reached at" then he will not receive the messages sent to that number.

I may have misunderstood the Author, but as I understood it he did not want to receive messages from he iPhone on his Mac. But it's good to have more options.