Remove the user look for entry from the owner lookup in MS CRM 2015

Hello,

We have a requirement where we have to disable the USER look for in the owner lookup

On case entity, if we click on owner lookup, the pop up window will show both Team and user to select, but I need to remove the User entry. In crm 2011 we did this using crmForm functionality, but this is deprecated in CRM 2015 online.

So any suggestions. Thanks in advance.

Regards,

Siva