You can either remove users from individual workspaces or from your account entirely.

You will need to have Workspace Owner permissions to remove users from a workspace.

Navigate to workspace Options>Users. You will see a list of everyone who has access to the workspace. To remove users, simply click on the 'Remove' button.

NOTE: This will only remove the user from this workspace, not from the account.

You will need to be an Administrator to remove users from your account. When you remove users from your account, you will automatically be removing them from all of the workspaces that they are a member of as well. To do this, navigate to Account Settings>User management.

You will see a list of all users, simply find the user in question and then click on the 'Options' button and select 'Delete' from the drop down menu.