To Restrict a user in picker field -User Should not be displayed in suggestions

Hi Team, 

I have a requirement where I should not display test.admin user in suggestion

This is my current condition which I have implemented to restrict a user, but it still displays the user in suggestion

Users from Appian Directory, ri!user : User (Type)

a!pickerFieldUsers(
label: "Select User",
maxSelections: 1,
value: if(ri!user = "test.admin", {}, ri!user),
saveInto: ri!user,

required: true,
),

Could someone please help us on this.