when set disable RowSelectors: 

Hello,

In general there should not be such change in the header, if only the row selectors are enabled.
The expected behavior when row selectors are enabled could be found at our online samples:
http://es.infragistics.com/samples/aspnet/data-grid/row-selectors 

Are you by any chance using Multi column headers?
For reference: http://es.infragistics.com/samples/aspnet/data-grid/multi-column-headers

If you continue experiencing difficulties removing it, could you share the markup and CodeBehind code, in order to further look into this?