Members can remove themselves by clicking on "Leave club" in the club info tab (on the web) or by tapping "leave this club" in the club settings (in the app), but admins can remove a member this way:
On the members tab of your club page, click edit:
Then click on the check box(es) next to the member's name you want to remove--and click save:
The system will ask --are you sure? Click yes.