Unfortunately, according to the Oaisys administration guide once an Oaisys account has been created thru Active Directory integration it cannot be deleted. You can set the account to expire on a specific date/time, effectively disabling the account. However, you should still be able to modify associated extensions by highlighting the user and clicking edit at the top of the page. It is possible that the account you are using for administration does not have permissions to modify these particular users. Check what users/groups your account has access to under Users and Permissions > Your account or the group your account is a member of > Application Permissions > Admin > select the "Users" tab or "User Groups" tab.
