Try sonmething like this:
<Style x:Key="DataPointStyle1" TargetType="{x:Type dvc:LineDataPoint}">
<Setter Property="Visibility" Value="Collapsed"/>
</Style>
(dvc is your xmlns namespace definition for the datavisualization control.
If you want to make this available for all line series all the time, put this style definition in app.xaml.
<dvc:Chart>
...
<dvc:Chart.Series>
<dvc:LineSeries DataPointStyle="{DynamicResource DataPointStyle1}" ...>
...
If you don't want to bother with specifying the data point style in the chart, remove the x:Key element in your custom style, and it will be applied to all line series by default.