It is called a tab and you cannot delete it as such. What you can do is make the form popup and/or modal (which means it will open in it's own window) - find these on the forms properties 'Tutte' tab
Alternatively, if you don't want these tabs on all forms or reports, you can go into File>Options>Current Database and a few rows down, either untick the 'display document tabs' option or select 'Overlapping Windows', then click OK at the bottom - you will need to close and reopen Access for these to take effect.