Hi Marvin,
You can delete an internal ViEmu mark with :delmark, but that doesn't remove the VS mark. To remove the VS one, you need to use VS's 'toggle mark' command (Ctrl-F2 in my configuration). To find out what the keybinding is in your case, you can go to 'Tools|Options|Environment|Keyboard' and type 'Edit.ToggleBookmark' and see the assigned shorcuts (and possibly modify them if you want).
Unfortunately, it's pretty difficult to bind ViEmu's marks tightly to VS marks. The right solution would be to actually visualize ViEmu's internal marks somehow, but that involves some Major VS Surgery which is out of reach for the current version.
Regards,
Jon