I have certain pre-defined roles in my project like Project manager, Test Lead etc. There is a certain category of workitems which need to be assigned only to a user with the role of the project manager. keeping this objective in mind created a value set (role based userlist) under attribute customization to restrict the ownedby attribute  to the list of the users who are assigned with the role of Project Manager. When i implement this condition when i checked on the web / eclipse UI it was showing the filtered list of project managers however there was also an option "more" which was allowing me to assign this workitem to anyother role. 

Is it feasible to restrict a user from assigning the workitem to any role other than Project Manager.