To remove members from your group, you would need to Tap the 👤 icon from within the group.
Person Icon on Group home screen
Once you do so, you will reach the Group Members page, as shown below.
Group home screen page
To remove any member just tap on their name and swipe left.
Group home screen swipe left
At this stage, you will receive a confirmation pop-up.
Member removal confirmation
You can select the "Disallow rejoining" option to disallow the member to rejoin using the Join code.