[rt-users] requestors filling up the owner dropdownbox

[SB]

Hi,

I just started using RT 3.8.6 and i noticed that my ticket owner 
dropdown box is clogging up
I expected only the staff members to be in that list but now every 
customer dat creates a ticket ends up in this list
How do i stop that from happening ? & How do i clean that mess up?

Thanks

Leon Berkers