Note: You may only do this if you have the owner/moderator permissions in your channel.
Select the channel you wish to remove a user from.
Click on the "Members List" button at the top right corner of your workspace.
Select the user you'd like to remove or you can type their name in the "Search by Username" bar.
Click the vertical three dots that appear under the user's profile picture.
Select "Remove from Room".
Confirm "Yes, remove user!" to remove the user from the channel.