A Form used as a MdiParentForm cannot have 'AutoScroll set to 'false: at design-time, if you set its 'AutoScroll to 'true, it will automatically set the the 'IsMdiContainer property to 'false. Setting 'IsMdiContainer to 'true will automatically set the 'AutoScroll property to 'false.
The only source of the Scrollbars you are seeing must lie in the fact that your MdiChildForm has 'AutoScroll set to 'true, and the child Form has content that exceeds the Bounds of the Form when it is displayed Maximized in the MdiParentForm.
_manageUsers.AutoScroll = false;
to your code that creates the child Form.