You can stop the tick mark lines from being drawn on an axis by altering the axis' TickStyle object, for example:
This stops the major tick mark lines from being drawn. There are a number of other options available on the TickStyle object too such as prevent the tick mark labels from being shown. For more information see TickStyle.
Note, you will need to call shinobiChart.redrawChart() after you have made the changes to the TickStyle object in order for them to be visible.