problem with implicit schema creation

Hi All, I have a small problem with implicit schema creation. I have several members in my dev team that are member of a group (domain\devgroup). The problem is that all of these group member but one when creating a table or an object use the dbo schema where this one user when logs in and creates any object creates a schema and a user for himself in the database (this one user is not explicitly define anywhere on the server or on the database). Anything I can do other than asking him to qualify his create statements? Still this begs the question, why only one? Regards, Waqar