Hi @cddwyer
I don't believe there's an API call for removing users from Shared iPads - hopefully, someone can prove me wrong.
An alternative solution would be changing the number of users your device's cache. This is set in the prestage as per the screenshot below.

The number of users is not the issue - it's that the oldest users are not being bumped. Last year this feature worked - the oldest user would get bumped and a new user took take the 10th spot. It is getting increasingly frustrating with our 200 shared iPads starting to fill up.
From Apple documentation:
“When setting up Shared iPad with MDM, you can configure the number of students that can have cached data on a device. Shared iPad divides the available storage for each user after allocating sufficient space for iOS and apps. Space is equally divided by the number of defined users. For example, if Shared iPad is configured for five students and a sixth student signs in, the student with the oldest data is removed to make room for the new student to store data.
”
Excerpt From: Apple Education. “Education Deployment Guide.” Apple Inc. - Education, 2016. iBooks. https://itunes.apple.com/us/book/education-deployment-guide/id1092741957?mt=11