A User can be removed from User Groups through the Web Portal > Users > View User > Edit the User that you would like to make the changes to > deselect the User group/s that you don't want them to have access to > Save.
This can also be done through the individual group on the Web Portal > Users > Groups > edit on the group that you would like to make the change on > deselect the User/s that you don't want to be in the group > Save.