If you're looking to delete a user from a workspace or stack, please open the 'Share' model.
For Workspace:
Open the Workspace 'Share' Model and hover on the user name, click on the 'x' sign right next to their name and confirm. The user will be removed from the workspace.
For Stack:
Open the Stack 'Share' Model and hover on the user name, click on the 'x' sign right next to their name and confirm. The user will be removed from the stack.