How to remove a user from role mapping

I don't believe you can use a DELETE call to remove a user from a role mapping.
As I am not 100% sure on your approach the following could help you;

  1. You make use of role mappings to define which roles a user gets and have multiple users listed in the mapping.

  2. You add roles directy to a local user.

In the first scenario you can use two api calls to remove the required user: first you get the current implementation which you mutate by removing the desired user(s) and then you update the role mapping with the new definition.

In the second scenario you follow the same approach by retrieving and updating but now on the user itself.