If you want to deactivate a group, you have to be the admin of this group.

To deactivate a group that you no longer need, open the group overview via the menu at the top right (three lines).

Click on the group you would like to disable, then click Settings. Here you will find the option "Deactivate group". 

Note: It is not possible in Sdui to completely delete a group. Groups that are no longer required can only be deactivated.