Use the following. If you were to put this in a new workbook. Then what it will do is hide all the rows after 50. (grey them out)

Public Sub hide()
Dim r As Long
Dim wks As Worksheet
Set wks = ThisWorkbook.Worksheets("sheet1")
For r = 50 To 65536
wks.Cells(r, 1).EntireRow.Hidden = True
Next r
End Sub
If your using office 2007 then replace 65536 to whatever the number of rows it can store.