How do I secure my messages after syncing with another person's Mac?

"How do I secure my messages after syncing with another person's Mac?"

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Secure your Info:

If concerned for account access, then (A)verify your devices, and (B)modify your credentials. Open a new eMail address and using that for your Apple ID is best, along with a new password.

1. Verify your Devices:

Verify that all devices are logged in to your account, by using this link: Check your Apple ID Device List to See where you're Signed in - Apple Support

then...

2. Modify your Credentials:

Having been stollen, someone lily has your credentials. So, by all means, modifying them.

• Modify: your login credentials (Using Both a New eMail and a New Password for each and all sites)
• Associate: a different method of payment to your account
• Enable: Two-Factor Authentication for Apple ID and for all other accounts that have such an applicable option

For Reference:

Reporting Items:

You can report phone call's and messages info to Apple and others. Use my User: Using the Photos App to Report Scams - User Tip

Try changing your apple id password