How do you remove a role after it's been assigned to a user?

Hello,

I created a role and grant with User Managment responsibility.  I assign this role to say 5 users (FND_USER) also using User Management responsibility.  Later, management says, user#5 should not have this role.  How do I remove the role from user#5 or prevent user#5 from using the capability of the role?  On my system, when I query the role for the user, the "remove" icon is greyed out.  I'm on EBS 12.1.3 and DB is 11.2.0.2. 

thank you.

Ron