how to write a vba code to remove and replace UTF8-Characters

You can replace any chars that are out of e. g. ASCII range (first 128 chars) with placeholder using the below code:

Option Explicit

Sub Test()

    Dim oCell As Range

    With CreateObject("VBScript.RegExp")
        .Global = True
        .Pattern = "[^u0000-u00F7]"
        For Each oCell In [A1:C4]
            oCell.Value = .Replace(oCell.Value, "*")
        Next
    End With

End Sub

See this question for details about using Regular Expressions in your VBA code.

Then use regular expressions in a function like this one to process strings. Here I am assuming you want to replace each invalid character with a placeholder, rather than the entire string. If it's the entire string then you don't need to do individual character checks, you can simply use the + or * qualifiers for multiple characters in your Regular Expression's pattern, and test the entire string together.

Function LatinString(str As String) As String
    ' After including a reference to "Microsoft VBScript Regular Expressions 5.5"
    ' Set up the regular expressions object
    Dim regEx As New RegExp
    With regEx
        .Global = True
        .MultiLine = True
        .IgnoreCase = False
        ' This is the pattern of ALLOWED characters. 
        ' Note that special characters should be escaped using a slash e.g. \$ not $
        .Pattern = "[A-Za-z0-9]"
    End With

    ' Loop through characters in string. Replace disallowed characters with "?"
    Dim i As Long
    For i = 1 To Len(str)
        If Not regEx.Test(Mid(str, i, 1)) Then
            str = Left(str, i - 1) & "?" & Mid(str, i + 1)
        End If
    Next i
    ' Return output
    LatinString = str
End Function

You can use this in your code by

Dim cell As Range
For Each cell In Range("A1", Cells(Rows.Count, "A").End(xlUp))
    cell.Value = LatinString(cell.Value)
Next

For a byte-level method which converts a Unicode string to a UTF8 string, without using Regular Expressions, check out this article