Denying a group from setting an owner

Hello,

I’ve been trying to setup group permissions to a group, that will be able to open a new ticket, but not to set the owner.
I’d like it to remain Nobody (default).

I’ve granted the group with CreateTicket, but it seems to have given them the permissions to set an owner as well.
Any ideas how can I revoke this possibility?

Thanks,

Karmi.