für Spalte A
Option Explicit
Sub mat()
Dim LoLetzte As Long
Dim LoI As Long
LoLetzte = IIf(IsEmpty(Cells(Rows.Count, 1)), Cells(Rows.Count, 1).End(xlUp).Row, Rows. _
Count)
For LoI = 1 To LoLetzte
Cells(LoI, 1) = Application.Substitute(Cells(LoI, 1), Chr(10), "")
Next LoI
End Sub