Open a web browser and navigate to the "Gies Business - Mailing Lists" web interface (https://lists.giesbusiness.illinois.edu) 
Select “Campus Login” (top-right corner)

Log in using your "Campus AD Credentials"

Select the “Search for Lists(s)” or “My Lists” (bottom center) to find the specific mailing list you want to modify

Search for list (if not using "My Lists" Navigation)

Select on the list name to access its management page

Select the "Subscribers" tab (left-hand navigator)

On the Manage list members page, scroll down to the "Search for a User" field and provide part or all of the subscriber's netid in the “Search for a user by email address, name or part of them:” field and select "Search"

Locate the User(s) in the "Subscriber Table" and select (check) the checkbox next to the user(s) you want to delete

Select (check) the “Quiet (don’t send deletion email)” checkbox

Select "Delete selected email addresses" to complete the process

Once selected, the User(s) should no longer be listed in the "Subscriber Table"