In order to add/remove a user from iRODS, you need to have an account with type "rodsadmin".
If you do not know the type of your account, you can discover it with this command:
> iuserinfo myusername#myzone
A user can be removed only if there is no data stored in the user's home folder in iRODS, because the home folder is removed together with the user.
Once the home folder is empty, you can remove the user with this command:
> iadmin rmuser myusername#myzone