You cannot delete a group on Viadeo, but you can change its visibility:
Click Engage on the navigation bar, and then Groups.
Click Manage Groups
Click Manage next to the group you want to edit.
In the left-hand menu, click Group Settings.
Under Access, click Edit.
Choose one of the visibility options for your group:
Tip: if you want to delete a group because you don’t have time to manage it, consider asking another member with similar interests to take over management of the group.