You have two mistakes here; the first is that the parameters being added need to be within double quotes (as the prior answers have pointed out).
The other is that the OleDB database provider does not support named parameters within the query itself.
OleDbCommand.Parameters Property (System.Data.OleDb) | Microsoft Docs[
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OleDB Documentation
The OLE DB .NET Provider does not support named parameters for passing parameters to an SQL statement or a stored procedure called by an OleDbCommand when CommandType is set to Text. In this case, the question mark (?) placeholder must be used.
Try these changes to your problem
string query = "DELETE From AktiveUser WHERE ClientTyp= ? AND MAC_Adresse = ? AND User_Kennzeichen = ?";
OleDbCommand olecmd = new OleDbCommand(query, conn);
olecmd.Parameters.AddWithValue("@cltyp", rowview["ClientTyp"].ToString());
olecmd.Parameters.AddWithValue("@macadress", rowview["Arbeitsplatz"].ToString());
olecmd.Parameters.AddWithValue("@userkenn", rowview["Anmeldung"].ToString());