How do I logout from any other devices on iCloud?

You might want to consider changing your password and making sure it is a strong one. You can also add 2 Factor Authentication to your account for more security. If the attempts continue, you might want to consider a name change.

Link to contact Apple Support in the article.

Apple ID has been compromised.      *

Security and your Apple ID.   *

Checklist: If you want to see if anyone else has access to your device or accounts  *

 Apple ID Password Change.  -

ASC YouTube for How to reset your Apple ID password

You can change your Apple ID email address on your current Apple ID so you retain access to your data and prior purchases. Note the restrictions in the article.

Apple ID Name Change.  * 

Make sure you turn off Find My Phone before signing out of the old ID. Once signed out and Find My Phone is off, then sign in with the new ID, then turn Find My Phone back on.

 What to do after you change your Apple ID or password - Apple Support  *  I suggest doing this before you change your Apple ID or password.

Apple ID -Two-factor authentication   *

ASC YouTube for what is two-Factor Authentication

Two Factor Authentification availability.   *

2 Factor Authentication - How to generate a recovery key.  *

Apple ID - Using app-specific passwords.   *

Apple ID - Two Step Verifications FAQ    *