Go to the menu View and uncheck the option Schematic Tabs please. Note that it only hides the tabs, and if you open the next sheet, the previous one is not automatically closed, but is hidden behind. You can see which sheets are opened in the Window menu.
Thanks for the hint.
Is there a way to show only the sheetname (CPU, Power, IO,...) in the tabs? The tabs are to small for our designname+sheetname.
Unfortunately such customization is not available now. I will take care that it is added in a future release, since it sounds like a good suggestion indeed.