Customize Sharepoint App Bar, Hide Create Button

Noname320 I don't think currently there is any SharePoint out of the box way to customize SharePoint app bar to hide/limit any available options OR customize the behavior of those options.

Maybe, you can limit site creation by following: Manage site creation in SharePoint 

Also, Microsoft no longer allowing customers to disable the SharePoint app bar. Check below documentations for more information:

1. Introducing a SharePoint app bar that features global navigation 
2. Enable and Customize Global navigation in the SharePoint app bar 
3. Use the SharePoint app bar 
4. Introduction to the SharePoint app bar 

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