It is possible to change the symbol by editing the Windows registry.
So have it. It works on my units--if you use a symbol that the unit supports.

Only the zumo 3x0 series and certain nüvis support shaping points. Every other Garmin device treats them as regular via points.
You can however replace the via point symbol on the device with another, less obtrusive symbol: put an alternative symbol with the name "Pin, Blue.bmp" in the folder Garmin/CustomSymbols on the device. You can find one here: http://www.javawa.nl/viatoestel.html . The page is in Dutch, but the link to the file is easily found.
This doesn't hide the via point names though, and you will still be alerted when you reach a via point.