I think you may be referring to the Record Selector. To get rid of this, go into the form's properties, in design mode, and set the Record Selectors to NO.
Hi stoka, Possibly its not a vertical line but a margin issue, try going into design view and select all "Cntrl A" and move everything to the right to confirm that a field is not to far left and if it is a line you should be able to see it then.