Try this:
Sub RountIt()
Dim c As Range
Application.ScreenUpdating = False
For Each c In Selection
If IsNumeric(c.Value) And c.Value <> "" Then
If c.HasFormula Then
c.Formula = "=ROUND(" & Mid(c.Formula, 2) & ",0)"
Else
c.Value = Application.Round(c.Value, 0)
End If
End If
Next c
Application.ScreenUpdating = True
End Sub
Alternatively, simply set the number format of the cells to Number with 0 decimal places.