How to remove a Company Admin role via REST API ( 2017) ?

Hello,

I'm trying to remove a Company Administrator role via REST API (iMIS version 20.2.66.1216).

I tried making a PUT request to "api/GroupMember/GroupMemberId" (for example: "/api/GroupMember/ORGRELATIONSHIP-311533:311576" ). However, I got 400 (Bad Request). After inspecting the error, I got this Stack Trace:

[HttpException (0x80004005): A potentially dangerous Request.Path value was detected from the client (:).]
System.Web.HttpRequest.ValidateInputIfRequiredByConfig() +11790877
System.Web.PipelineStepManager.ValidateHelper(HttpContext context) +54

I also tried URI encoded path "/api/GroupMember/ORGRELATIONSHIP-311533%3A311576" and got the same error.

I have 2 questions at this point.

Can "api/GroupMember/GroupMemberId" endpoint be used to remove Company Admin role by doing PUT request ?
How can I solve the issue I described above?

Thank you for your help!

Best regards,
Sergii