How do I remove a user from a group?

You can use gpasswd:

# gpasswd -d user group

then the new group config will be assigned at the next login, at least on Debian. If the user is logged in, the effects of the command aren't seen immediately.

On Debian, the adduser package contains a deluser program which removes a user from a group if you pass both as arguments:

deluser user group

If your distribution doesn't have adduser, you can edit /etc/group and /etc/gshadow manually.

vigr
vigr -s

usermod -G "" username

removes all secondary/supplementary groups from username, leaving them as a member of only their primary group. this worked in Solaris 5.9