FYI, Emma from Tableau Support has came up with a reasonable 'hack' around this problem:
"""For this particular case, a workaround we can consider is to set the color of this single point to the same as the background, like black. Since there are no white and black in Tableau's built-in color palette, we should create a custom palette.
+Create a xml file with below content and save it as "Preferences.tps" under the directory: C:\Users\yourid\Documents\My Tableau Repository.
<color-palette name="My Own Palette" type="ordered-sequential" >
+Restart Tableau Desktop 10.0 to make the above tps file effect.
+In the set of marks for line, change SUM(count) from continuous to discrete then click "Color"-Edit Colors
+In the pop-up "Edit Colors[count]" window, select "My Own Palette" from the bottom of "Select Color Palette" drop down
+Click "Null" from the left panel, and select black from the right panel then click OK
+Now we can see that the point looks "disappeared"
+Click Null from the color legend of SUM(count) and select hide
Please see the attached workbook. It's not a good solution because we actually did not remove the point. Instead we just make it invisible. Actually we don't have a 'perfect' solution based on the current data source and view.
Hope it helps a bit. For more information about how to create a custom color palette, please see below article.
Create Custom Color Palettes