Hi @tist
In v3.3.0 that will be released on August 16th, team and system admins will be able to remove members from a team via the "manage members" modal or via the system console.
As for revoking an invitation to a team - you could reset the "email invite salt' in "system console" -> "security" -> "sign-up" but this might reset the salt for the server and not just the team. Alternatively, if the user does join the team, you can make them "inactive" via the "system console" -> "teams" -> "users" which will block them from signing in. Once v3.3.0 is released, you will be able to remove them from the team.