How To Restrict Users From Logging More Than Once In Excel

I am doing an excel survey using forms. I want to restrict users from completing the survey more than once. i am using below code to input user login details in to a spreadsheet but i am not sure how to restrict them. I want a msgbox or some sort and then it closes the excel active workbook.

code to get username:
Sub GetName
Dim r As Range
Set r = Sheets("UserNames").Range("A" & Rows.Count).End(xlUp).Offset(1, 0)

r.Value = Environ("username")
r.Offset(0, 1).Value = Date
r.Offset(0, 2).Value = Time

End Sub

After looking through various forums i found code to highlight duplicates but not sure how this can be modified to suit my need.

Code to find duplicates:
Sub Dups()

Application.ScreenUpdating = False
Sheets("UserNames").Range("A1:A500").Select
Rng = Selection.Rows.Count

[Code] .......