Whenever you have 1:N relationship with systemuser entity, you will not be able to delete or remove the systemuser record from Users associated view. It is by design. So to remove that you must have a lookup attribute of related entity on your systemuser form and need to delete that record from lookup and save the form. Let's understand with an example.
Suppose I have 1:N relationship between Marketing List and systemuser entities. So now I have Users associated view in left navigation of Marketing list record. Let's add an existing user for this marketing list record. There is no direct way to remove, delete or exclude the added user record. So for deselecting this user record we could have a lookup of Marketing list entity on systemuser form. Add the Marketing list lookup attribute on systemuser form, publish it. Now back to Users associated view, double click the user record, delete the marketing list record from the lookup on user form. It will refresh the parent grid and removes the user record.
I hope this will be helpful.
